How to determine QPSK slopes?

I get some data back from the PO-32 Tonic backup signal with a dumb BPSK decoder. However, people online are saying that the signal resembles a QPSK. I think I can adjust my rouge program to do this. But I am not sure how to determine the slopes for the QPSK 00, 01, 10, 11 bit code.

Just trial and error?

Some previous discussions on that:

Have you tried to visually inspect the signal to get a feeling for the modulation type?