The pinch test has been around for years doing excellent work for everyone who learns to do it.

With a warmed up engine, run it WOT to clear it out. Go to idle and let it settle out on it's own. Set the idle trim to get a low enough idle that the airplane will jsut sit there. And if you have to reset the trim, start over with the clearing out etc.

When it's idled 8-10-15seconds or so, pinch the fuel delivery hose up near the carb. Lots of guys use a hemostat or needlenose pliers to do this. Pinch and hold and listen.

If the engine speeds up and dies, the low speed was too rich. If it just suddenly dies, it was probably too lean.

Until you get some experience doing this, adjust the low speed needle just about 1/8 turn each change. Always test right after, and start from the WOT clearing out.

One detail I've seen overlooked is to verify that the carb actually has a low speed needle and isn't an airbleed carb. Airbleed is a bit more complex in how you change it's setting.