Oil, coolant and fuel each have a distinct smell. The colour of the smoke is not a very good indicator, though.
Agreed. Oil and being rich during idle can look the same, and smell the same.
It's only when something's REALLY gone that you can tell the difference with the smoke, however by this point there are other factors to confirm this.
A great way to check valve seals if you don't have a leakdown tester is to have someone follow you down a hill. Put it in 3rd and leave the clutch out and coast down the hill. When you get to the bottom, punch it. If a good grey/blue puff of smoke comes out, it's an indicator. The vacuum created when coasting (healthy motor sees upwards of 26in/hg) will suck in the oil past the valve seals if they are leaking.
