frames of blockstun on lv1, 2, and 3 moves are 12f, 15f, and 18f respectively (the first frame starting on the frame the move connects)
so i think if you have a move you can cancel into that's 12-14f and/or one that's 15f-17f you can simply cancel into it from the move you're trying to find out about while the dummy is on all guard and recover a/b/c/d and see if they exit blockstun or stay in it to figure out if it's lv1, 2, or 3 then from there you can use the recovery+active frames to figure out how + or - the move is on block
this would only work for some moves though since you need to cancel out of it to find out what lv it is
if you already know what lv the atk is then you should know the blockstun already and be able to find the +/-