After you are done racing or driving look at your glow plug. I usually can tell by ear but that come with experience. As nitroxray said if you nose over or cannot richen your engine enouh then you are too lean or have an air leak somewhere. Check fuel line, exhaust seals and your carborator. Overly wet spots indicate leakage.

For checking the glow plug...
If it is distorted or melted at all then yu a running to lean. You will also notice some melting of the piston if this is noticed and the condition is really bad.

If the plug has a heavy coat of oil on it then you are burning rich. As said before if fuel is noticed coming out raw from the exhaust then you are very very rich and should continue slowly leaning it out. This condition coyld also be caused by a bad filter or a worn out glow plug.

If the plug has none of the above and just a slight browning of the first portion of the wire then its rught where it needs to be.