Trial and error.
Also examining the monster database to see what existing monsters are using is very helpful.
<edit> list from one of my databases.
Code:
1 Pheasant
2 Turtledove
3 Robin
4 Apparition
5 Poltergeist
6 WingedSnake
7 Bandit
8 Ratling
9 FireSpirit
10 Macaque
11 GiantApe
12 ThunderApe
13 Snakeman
14 SandMonster
15 HillMonster
16 RockMonster
17 BladeGhost
18 Birdman
19 HawKing
20 TombBat
21 FlyingRooster
22 BlueCooer
23 GameBird
24 WarGhost
25 BattleGhost
26 FireSnake
27 CateranBoss
28 BlueElf
29 ElfKing
30 SmartMonkey
31 RoaringApe
32 SnowApe
33 CrazySnake
34 SandDevil
35 MountMonster
36 StoneMonster
37 BladeMonster
38 BirdKing
39 SilverHawk
40 Robber
51 MonolithMonster
55 BanditL97
56 BloodyBat
57 BullMonster
3120 GuildBeast
59 RockMonsterL15
64 HeavyGhostL23
65 WingedSnakeL28
66 BanditL33
67 FireRatL38
68 FireSpiritL43
69 MacaqueL48
70 GiantApeL53
71 ThunderApeL58
72 SnakemanL63
73 SandMonsterL68
74 HillMonsterL73
75 RockMonsterL78
89 DevilKing
77 BirdmanL88
78 HawkL93
79 BanditL98
80 TombBatL103
81 BloodyBatL108
82 BullMonsterL113
83 RedDevilL118
84 Banditti
85 SeniorBandit
86 VampireBat
87 VampireKing
88 BloodyDevil
76 BladeGhostL83
90 FlyingBat
91 BloodyKing
92 BullDevil
93 GreenDevil
100 Skeleton
101 GhostWarrior
102 SkeletonKing
103 GhostAres
105 RockMonsterL17
106 RockMonsterL42
107 RockMonsterL72
108 RockMonsterL92
110 WindSnake
111 AlienBandit
112 FireRat
113 FireSpirit
120 MinMacaque
121 ElfApe
122 SlowApe
123 SnakeMonster
124 SandElf
125 LapidatingMob
126 ToughMonster
127 EvilBlade
900 Watcher1
910 Watcher2
911 Patrol
2000 Robot
2001 Robot
2002 Robot
2003 Robot
2005 WaterElf
2006 WaterElf
2007 WaterElf
2008 WaterElf
2009 WaterElf
2010 Robot
2011 Robot
2012 Robot
2013 Robot
2020 DivineHare
2021 DivineHare
2022 DivineHare
2023 DivineHare
2024 DivineHare
2030 NightDevil
2031 NightDevil
2032 NightDevil
2033 NightDevil
2034 NightDevil
2040 WaterElf
2041 WaterElf
2042 WaterElf
2043 WaterElf
3000 OddMonster
3001 NorthMonster
3002 SouthMonster
3003 WestMonster
3004 EastMonster
3005 FuryGhost
3006 CrazyGhost
3007 RoaringGhost
3008 ScaryGhost
3009 BloodGhost
3010 IronBird
3011 CopperBird
3012 SilverBird
3013 GoldenBird
3014 FastBird
3015 BrutalBandit
3016 WildBandit
3017 CruelBandit
3018 SingleEye
3019 TyrantBandit
3020 AberrantEvil
3021 CecropsBoss
3022 Minotaur
3023 Jumpy
3024 EvilMonkey
3025 Pan
3026 EvilHawk
3027 GlumMonster
3028 ShakeMonster
3029 IronEagle
3031 Cateran
3032 PoisonousRat
3033 StoneBandit
3050 HeresySnakeman
3100 FireSnakeSpirit
3101 ToughSnake
3102 SnakeKing
3103 WaterSnake
6000 ToughHorn
6001 ToughHorn
6002 ToughHorn
6003 ToughHorn
6004 ToughHorn
6005 ToughHorn
6006 ToughHorn
6010 LonelyTyrant
6011 Ghost
6015 MonkeyMonster
6016 Cecrops
6017 ApeMonster
6018 Python
7000 DevilGuard
7001 WarriorDevil
7002 TrojanDevil
7003 ArcherDevil
7004 FireDevil
7005 WaterDevil
8100 HumanMessenger
8101 WingedSnakeMsgr
8102 BanditMessenger
8103 RatMessenger
8104 ElfMessenger
8105 MonkeyMessenger
8106 GiantApeMsgr
8107 ThunderApeMsgr
8108 SnakemanMsgr
8109 SandMonsterMsgr
8110 HillMonsterMsgr
8111 RockMonsterMsgr
8112 BladeGhostMsgr
8113 BirdmanMsgr
8114 HawkMessenger
8115 RobberMessenger
8116 BatMessenger
8117 BloodyBatMsgr
8118 BullMessenger
8119 DevilMessenger
8200 HumanAide
8201 WingedSnakeAide
8202 BanditAide
8203 RatAide
8204 ElfAide
8205 MonkeyAide
8206 GiantApeAide
8207 ThunderApeAide
8208 SnakemanAide
8209 SandMonsterAide
8210 HillMonsterAide
8211 RockMonsterAide
8212 BladeGhostAide
8213 BirdmanAide
8214 HawkAide
8215 RobberAide
8216 BatAide
8217 BloodyBatAide
8218 BullAide
8219 DevilAide
8300 GhostKing
8301 WingedSnakeKing
8302 BanditKing
8303 RatKing
8304 ElfBoss
8305 MonkeyKing
8306 GiantApeKing
8307 ThunderApeKing
8308 SnakemanKing
8309 SandMonsterKing
8310 HillMonsterKing
8311 RockMonsterKing
8312 BladeGhostKing
8313 BirdmanKing
8314 HawkBoss
8315 RobberKing
8316 BatKing
8317 BloodyKing
8318 BullKing
8319 EvilLord
58 RedDevilL117
200 DevilKingL120
201 DevilKingL122
1400 HugeSnake
1401 BanditLeader
1402 HugeSpirit
1403 CateranSoldier
1404 SeniorCateran
1405 CateranLeader
1406 ChiefCateran
1407 HugeApe
1408 SeniorApe
1409 AlienApe
1410 SeniorSnakeman
1411 Serpent
1412 SeniorSerpent
1413 AlienSerpent
1414 Basilisk
8415 MeteorDove
8416 MeteorDove
8417 EvilGhost
8418 NightDevil
8419 WaterDevil
8420 WaterDevil
8421 WaterDevil
8422 WaterDevil
9028 SkeletonD
9027 SkeletonC
9026 SkeletonB
9025 SkeletonA
9023 FireRatD
9022 FireRatC
9021 FireRatB
9020 FireRatA
9018 EvilBatD
9017 EvilBatC
9016 EvilBatB
9015 EvilBatA
9013 MagicBatBoss
9012 SwiftBatBoss
9011 FastBatBoss
9010 GhostBatBoss
9008 MagicBat
9007 SwiftBat
9006 FastBat
9005 GhostBat
9003 GoldGuard
9002 SilverGuard
9001 CopperGuard
9000 IronGuard
3141 Slinger
3130 GanodermaL72
3131 GanodermaL70
8500 WaterLord
3132 GanodermaL74
3133 GanodermaL76
3134 TitanL77
3135 TitanL75
3136 TitanL79
3137 TitanL81
3155 MinotaurL120
3156 Syren
3312 CuteBunny
3311 PlayfulBunny
3310 CuteBunny
1610 Goblin
3149 Talon
3148 FiendBat
3147 BlueBird
3146 NagaLord
3145 AgileRat
3144 Bladeling
3143 Gibbon
3142 GoldGhost
5045 UndeadSoldier
5046 Phantom
5049 UndeadSpearman
5050 Revenant
5051 Eidolon
5053 Temptress
5054 Centicore
5055 Wraith
5056 HellTroll
5057 Naga
5058 Syren
5059 UltimatePluto
5060 Wraith
3564 SandMonster
3561 Turkey
3562 800poundTurkey
3563 FireSnake
3560 StoneMonster
3709 MagentaRooster
3708 BlueRooster
3707 GreenRooster
3706 PurpleRooster
3646 FurySatan
3645 BeastSatan
3644 Satan
3643 Theodore
3642 Wayne
3641 Victoria
3640 Daphne
3639 Timothy
3638 Philip
3637 Peter
3636 Andrew
3635 CleansingDevil
3634 Banshee
3633 SwiftDevil
3632 HillSpirit
3631 FearlessBeast
3630 CrazyBeast
3629 SensitiveBeast
3628 AngryBeast
3627 FurySkeleton
3626 CarrionSkeleton
3625 LameSkeleton
3624 BoneSkeleton
3623 DemonBat
3622 ViciousBat
3621 CruelBat
3620 FuryBat
3619 HadesLord
3618 EarthHades
3617 WaterHades
3616 WoodHades
3615 FuryApe
3614 LightningApe
3613 RainApe
3612 WindApe
3611 FiendLord
3610 WhiteFiend
3609 RedFiend
3608 BlueFiend
3607 SerpentSpirit
3606 CryingSerpent
3605 RoaringSerpent
3604 SingingSerpent
3603 DemonLord
3602 AbyssDemon
3601 MudDemon
3600 ValeDemon
4032 ElfKing1
4033 BloodyBat1
4034 HillMonsterKin
4035 BirdKing1
4036 WingedSnakeKin
3320 KungFuBunny
3579 FishA
3582 BlueCock
3583 MagentaCock
3584 Siren
3585 BrownCock
3586 FishB
3039 SugarplumFairy
3037 FowlEmpress
3321 Piglet
4058 PinkPiglet
4060 PinkPiglet
4082 Vampire
4083 Vampire
4084 Vampire
4085 Vampire
4086 Vampire
4087 Vampire
4088 Vampire
9999 GuildPatrol