The more or less industry standard pressure angle in the UK is 20° and in the USA 14½° seems to have been favoured. The trouble is that there are all sorts of variations to this, some for entirely justifiable reasons, that can confuse the issue and to the Mk.1 eyeball there is little in the way of obvious differences.
You could however try this:
Firstly you need to know the DP of the gear. To find it add two to the tooth count and divide that by the O.D of the gear.
Take a measurement over any number of teeth with whatever equipment you have, multiply the cosine of the assumed PA by 3.1416 and then divide by the DP. Add this to your measurement, this should be the measurement over one more tooth if its not the same, change the PA and try again.
Example:-
10dp 14.5 pa, 30 teeth,
Measurement over 3 teeth .776
Cos 14.5° = 0.986147
(0.986147 x 3.1416) = 3.04
3.04/10 = 0.304
0.304 + 0.776 = 1.080 which should be the measurement over 4 teeth if the gear is 14.5° PA.
