I find them when I consider symmetry properties of the board. When you have a position and flip the board vertical you end up with a mirrored position that should have the exact same value. Whenever the position and the mirrored position values are different you know there is a bug somewhere. This way I eliminated most of the bugs. The last I found is caused by not properly handling under promotions.
Consider the following position
If white promotes the pawn to a queen it is mated next move, moving with the king is slightly better as White then is "Mated in 2". The best move however is e7-e8 N. Then black must move out of check and white gains an additional move. So this positions value is "Mated in 3" but the table shows "Mated in 2" because it does not consider the under promotion to a knight.
A nice shot of the inconsistency of the table.
So this is the next thing to fix and then hopefully the KQKP is ready and I can finally progress to the KPKP table.