Testing for heated headers with fingers is routine for me. And, I have never burned my hands/ finger in the process. Just a little knowledge about the human body reporting apparatus makes it quite safe.
You just need to understand how the body senses heat. The heat sense is a delayed report or an after-the-fact condition.
example: You can pass your fingers through open flame safely. They key is doing it with a short time frame(s). Your finger will feel hot after your have passed it through the flame. If you want a general hot feeling, repeat the action in short interval exposures. Your blood circulation is also a cooling system that will cool localized heated body parts and distribute that heat throughout the body. However, it has a limited heat distribution rate. This capacity must not be exceeded.
Head pipes can be monitored in a like manner. Always assume they are as hot as flame. Flash touch the pipe with very very short contact times. If they are hot, your fingers will feel warm after you have released the pipe. DON'T wait to feel the heat before removal! You can repeat the flash touch process with increasing frequency until your hand "averages" the heating effect that was felt. Each pipe can be tested with the same going-in assumption. "It's far too hot to hold until it is proven cold." If you can keep time tapping in beat to music. You then compare relative tapping/contact time and the heat felt on each pipe to the other pipes. If the engine is running, or trying to run, at least one pipe is hot/warm.
The only danger is false assumptions. Just because one pipe is proven cold, does NOT mean the next one can be grabbed with the same contact duration!
I don't appreciate the saliva approach for two reasons. One is steam, which by definition is 212 degrees F or higher, has a physical mass of it's own which remains in contact with the skin after removal from the heat source. This will blister skin faster than the application of dry heat which only has the mass of surrounding air that the body can more readily distribute/dissipate.
The second reason is Saliva residue and the suspended skin oils that can stain/mark chrome surfaces, which you later have to polish off.
I only use dry hands/fingers on chrome head pipes, since I'm too lazy to clean bikes with high frequency. But, someday, I'm going to spring for one of those IR probes, primarily because I'm a gadget/tool nut. Though I doubt I'll have it with me while doing a troubleshoot far away from my garage. Most likely I WILL have my hands and internal heat sensors with me, though, and enough brains/skill to employ them without self injury.
Cheers,
