You can work through the plug hole. You will need something to precisely measure liquid. Set the engine to top dead center. Fill the cylinder with engine oil to the plug hole, measuring exactly how much you put in. Call this measurement "A". Then turn it over to bottom dead center, and measure exactly how much additional oil it takes to reach the plug hole again. Call this measurement "B". Measurement "B" is also your displacement per cylinder. For example, the CB750's are actually 736cc four cylinder engines; 184cc per cylinder. So measurement "B" for a CB750 SHOULD be 184cc (or 184mL).
Compression Ratio = (A+B)/A : 1
So for example, on my 78 CB750F, if my measurement for A was 23mL, and my measurement for B was 184mL, then:
CR = (23+184)/23 : 1
CR = 207/23 : 1
CR = 9:1
Once you're done, draw as much oil as possible back out of the cylinder and expect some smoke until it burns the remainder off.