I have done two steering head changes 30 deg to 27 on a couple of bikes for better cornering speed.
This is the way i worked it out ,on my own throughts may be theres a better way..
To check your frame angle
I just get the bike complete wheels forks shocks and level it out on the ground useing a long straight edge and sprit level
Then i get a builders sliding bevel and put it agaist the forks you will have to put a sprit level on top of the bevel and level the bevel,
this will give you your deg angle........ then drop the bevel in a builders drop saw and rotate the blade to suit the bevel angle, the saw has degs numbers on it,
this will give you a deg angle within two degs on my sums,to your frame.
Im in the building trade so may sound funny to some but worked for me..................................