I'm going to be putting on a new tire and tube, so guess about the only thing I need to really check out is the spokes/rim strip.
The wife and I have been moving a lot of old furniture and stuff in and out of our big basement door. I try to make it a habit to sweep those areas with a large magnet when we're done. I have a feeling it's some kind of small furniture staple or brad. A lot of the rubber on my front tire is pretty thin: wouldn't take much to get through it.
Still, I'm curious what it is. If I can't anything will the naked eye, I'll try that napkin trick.
Although my spit trick didn't work, it could well be the valve stem, too. I check my tire pressures a lot, which increases wear on the valve and also increases the chances of something getting in there. I've probably checked the air pressure in that front tire hundreds and hundreds of times.