Although my main lump is a merc, I'm not totally clued up about the old smaller ones, however a couple of observations from the small / old end of the scale:
Telltales were a bit of a novelty even on larger engines - I have a '72 vintage 25 where you have to be very aware of the difference between steam & "cold start smoke" - there is a pitiful spray out the idle (top) exhaust port, and that is the tell tale. That flavour of engine was in production 'till 1975/6.
If your Merc is one of the later end of a 25 year production run from the 60s, there's a pretty high chance it won't have a telltale. The fact that it hasn't siezed yet might hint that the pump still works and it's one of them! There is a fab site for ancient OMC stuff, (old-OMC.de or something like that) but unfortunately nothing like it for merc / Yam etc)
The exhausts on older engines did tend to literally be a shaped pipe just behind the prop. As all the cooling water went down the exhaust for cooling, there's pretty much no way of knowing its flowing... until either the paint starts peeling form the leg or it siezes! There lies the hazards of small old engines.
Simple solution is preventative maintenance & change the impeller annually.
Depending on the height of your transom, if there is no "relief port" and you've perfectly tuned it in a bucket, remember to take a screwdriver with you on the first outing - a couple of inches difference in depth on the exhaust could be enough to stall it at idle, so you may need to up the idle speed.