First, erase any EVs via EV reducing berries.
Then, do an IV battle at level 100 on wifi.
Input Ho-oh's stats and nature in IV calculator.
Once you get accurate IVs, input Ho-oh's nature and IVs in IV to PID applet.
If the results are Method 1, Ho-oh is legal.
If the results are Methods 2/3/4 or no possibilities, Ho-oh is hacked.