RE: 180 Saito
I've tried this and you cannot tell by eye. You would need a digital depth gauge.
On the Saito 150, 180, & 220, I use a Exacto blade jammed in the gear teeth next to the cam cover when inserting the com cover on the crankcase to insure that the gear does not move when inserting.
As you say - after the cam is installed there no way to tell other than observing the valve train farther up.
One way is to measure the lifter height and you would have to measure them at TDC and then spin the crank one full turn and measure them again to make sure that you are measuring at the overlap interval.
Remember that overlap interval center point should be about 5 degrees before TDC.
You can assemble the rest of the engine and check yourself.
On the overlap stroke and at 5 degrees before top dead center, the rocker arms should be parallel with each other across their tops. One valve will be almost closed and the other almost open.
Also, you should be able to blow through the muffler and out through an open carburetor.