Reeds contribute to crankcase pressure, but I don't think they really affect cylinder compression.

If fuel is spitting back out through the carb throats, that's an indication of bad reeds.

Or if you have a fuel delivery problem and rule everything else out.

Otherwise, you'd have to pull them for visual inspection. They're usually located on the intake manifold.

The shop manual for your specific engine will give you the specs; if you don't have one and can't get your hands on one, usually you can just hold the assembly up & see if any light gets through. Realizing that serviceable reeds don't always block out 100% of the light.

Also check all the reed stops for consistent clearance. "Reed stops" limit how far the reeds are allowed to bend back when they open. Not sure what the tolerances should be -- if there's any variation, you probably want to replace the assembly.

If you find a broken reed, you absolutely must get the broken piece out of the engine (crankcase or cylinder) before it shreds your engine.