Input the Pokemon's accurate IVs and nature.
Interpret the results:
No possibilities: If Pokemon is legendary, it is fake/hacked
Method 1: Indicates a legendary Pokemon, a Pokemon from DPPt, or Emerald's non wild or soft resetable starters/fossils/kecleon/beldum etc
Method 2, 3: Indicates a wild, non legendary Pokemon from Gen 3 Games
Method 4: See above, just rarer
To summarize the methods:
Game || Wild || Non Wild
DPPT || 1 || 1
RSE || 1,2,3,4 || 1
FRLG || 1,2,3,4 || 1