In theory, the engine thrust line should go through the plane's neutral point, which is the point where all plane air forces can be centred (sigma M = 0)
Thust high located engines have some upthrust (see Catalina), and the lower the engine, the more the engine must be angled down.
Thus high wing trainers with quite low engines must have substantial downthrust, while low wing planes with high engines may be set neutral, or even have some upthrust.
Tests:
Test 1
Fly trimmed level at full throttle. Suddenly cut throttle. If the plane pitches up, there is too much downthrust. If plane continues flight path before the nose drops, thrustline is OK. If plane immediately drops nose (tucks under) there is too little downthrust.
Test 2
pull up plane to vetical at sufficient throttle for towering vertical. If plane tucks under when it slows down, there is too much downthrust, and vice versa.
Test 3
fly slowly at reduced throttle. Apply throttle. When the plane looses altitute before climbing again, there is too much downthrust, and vice versa.