Need opinion from those that KNOW.

I agree. You can find external residual valves, and some systems do fine without them. The reason for them is, that wheel cylinders can sometimes allow air to enter past the seals as things retract, and the purpose of the valve is to maintain a small positive pressure in the system so that does not happen.

I do not for certain know how you would tell a LACK of a valve, but if you are VERY CAREFUL, you could take a BLUNT probe, and GENTLY enter it into the disc side and find that you could easily push it through the fitting and down into the internal passage. If the drum side has a valve, there will be a small rubber diaphragm in there which will stop the probe. DO NOT use anything pointed and do NOT force it.