Agree with Chris. Pull your cowl and look at the rear of your block. At the top of both banks you will find a hose that connects between banks and then a hose leading down below the block from the Port bank. On the way down your Pee hose branches off.
My son's 150 had this setup and I thought it really SUCKED!!!!!! So I removed the pee hose from where it was, putting a union in the line where it was and looking on top of the block aft of the flywheel was another hose which went straight to the popoff valve below the engine. This hose had water in it whenever the block filled, regardless of the position of the tstat and you had pee flow within 30 seconds of starting the engine. So I put the tee in that line with the pee hose and solved the problem. I like that much better. Otherwise you just wonder if you have cooling or not and it will come and go as the stats open and close and all that "stuff".
On the visual indications, if you have water coming out of your prop hub while idling on the muffs, you have cooling water through the exhaust manifolds at least meaning the water pump is pumping. Also the amount of water excaping from around the cups of the muffs will decrease when the engine starts and the impeller in the water pump starts moving water.