Pinch test with the engine wide open and running smooth, you pinch the fuel line to the needle very quickly and release. The engine should speed up slightly. If too lean, when you pinch, the engine will sag slightly. Also if you have a typical trainer, the tank is too low, so you will want to set the engine richer yet. Tanks SHOULD BE mounted with the centerline no more than 1/4 to 3/8" lower than the needle valve. Most trainers fail this measurement, and you end up cooking the motors.

Idle, you set by how it responds to throttle. Usually, most engines will need to be leaned slightly after the engine break-in. Slightly is about 1/16 to 1/8 of a turn.

If you start having trouble with the idle, it could be the plug, or the engine is getting varnished and needs cleaning. Crock pot and antifreeze works the best. Look it up to find out more with the search function.