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

Onix
Onix 		Base
stats		 Stats Effort Stats Effort Possible genes (IVs)
Level Level
HP 35
83–96
19–20
Attack 45
43–57
24–26
Defense 160
142–155
13–14
Special Attack 30
30–44
3–5
Special Defense 45
43–57
27–28
Speed 70
65–78
21–22

Hmm, I need more information to figure out your Pokémon's genes exactly. Try raising it to level 44 and entering its new stats.
Remember: if your Pokémon battles at all, you'll need to track the effort it gains. Consider saving your game and using Rare Candy instead.

And for your copy/pasting pleasure:

19–20 HP; 24–26/13–14 Physical; 3–5/27–28 Special; 21–22 Speed

or start over