There's many ways to find your IVs (Or DVs, same thing) in GSC. But this would be the easiest:
DONT TRAIN IT
Rare candy to level 50/100
Compare to mon's Max wild stats at that level (www.pokefor.tk
's pokedex). Note: This is only accurate from Lv 50 onwards, I think, so doing it at lv 30 might get you wacky results
At lv 50: Subtract 1 from 15 for each point the mon's stat is lower than its species max at Lv 50 (NOTE: using the WILD part.)
Lv 100: Subtract 1 from 15 for each 2 points... etc.
The number you get in the end should be your DV
eg: I have a mew with the following stats after I rare candied to Lv 50/100:
since the species max for a wild Mew at lv 50 is :
Then, subtracting, I get...
Those would be the IV/DV of this mew (And in GSC, IV/DVs are 0-15, not 0-31)
Same thing, but each IV would control 2 points, so Max wild mew is
Then if I have 338 for HP, I have 14 for HP DV, not 13.
Note: You can also do this at max training, but if you have SOME training but not all, then it's bloody impossible to find your DVs, unless of course you know exactly how many stat EXP your mon has in each stat.