Question 578764
If its a regular polygon the total internal angles sum:

(S-2)180, where s is the number of sides 

for a triangle s = 3, rectangle s=4, pentagon s=5, etc

Once you have the sum, simply divide by S

If it is NOT a regular polygon, that is, the internal angles are not all equal, you must have some sort of reference

Otherwise you will have to use a protractor and estimate.