How to lean with constant speed prop on multi-engine aircraft without EGT
There is a big difference between "by feel" leaning of a carburated vs fuel injected engine.  Carburated engines have less even fuel distribution and when leaning, one cylinder, the already leanest one, will start to misfire quite a bit before the others.  So with a carb it's easy to detect one cylinder starting to misfire (the cylinder usually doesn't totally quit right away - you will pick up random misses in the vibration rhythm).

With a fuel injected engine the mixture distribution is more even and misfire of one cylinder will be accompanied by misfire of its sisters almost immediately.  So on an injected engine you will lean and suddenly the engine will really start to stumble, whereas with a carburated engine there will just be these random dropouts in the engine's "beat", getting worse as you continue leaning.

So if you are leaning on an aircraft with fuel injection it is really fairly straightforward.  The engine will go from smooth to pretty rough right away, whereas with a carb it takes careful attention to detect the early onset of misfire. You are at a disadvantage to some degree because the airplane is less familiar than to the instructors.  It's a skill you will pick up over time as you develop at ear for it.

Don't worry too much about it. Err on the side of smoothness, knowing that the difference in fuel consumption between perfect and good enough is negligible (and if you're paying hourly on a hobbs meter, there is no benefit to you anyway), and it's wise to err on the side that is beneficial to the engine (too rich).