One quick method that my instructor taught me was to run the engine at full throttle (airplane obviously restrained), then turn the high speed needle counter clockwise slowly until the engine goes from two stroke to four stroke (engine will slow RPM and make a distinctive change in the way it sounds). Turn the needle valve clockwise a bit and then quickly pinch the fuel line with your fingers. If the engine RPM goes up then quickly goes down once you remove your fingers, it is probably ready. Try this against the tachometer to see where that engine "note" changes then, eventually you will use your ear rather than the tach. It takes practice to get that right, but it works just fine. Not that the tach method does, not, just that it is an alternative to using the Tach.