WHIRLWIND EXPLANATION: Pokémon have a fixed, permanent score from 0 to 31 for each stat that affects how good that stat can ever get.

This calculator will figure out that score (called a "gene" or, more obtusely, an "IV") for you, so you can discard the unworthy. It's more accurate for higher-level Pokémon, so it's helpful to go into a level 100 wifi battle with someone and check your Pokémon's stats from there.

Stat calculator

Gengar
Gengar 		Base
stats		 Stats Effort Possible genes (IVs)
Level
HP 60
227–258
31
Attack 65
133–164
31
Defense 60
123–154
31
Special Attack 130
262–293
31
Special Defense 75
153–184
31
Speed 110
222–253
31

Congratulations, you've narrowed your Pokémon's stats down exactly!

This Gengar's Hidden Power inflicts Dark damage, with 70 power.

And for your copy/pasting pleasure:

31 HP; 31/31 Physical; 31/31 Special; 31 Speed

or start over