App or? Not sure on how to check temps.
Yes there is an app.....go over to alldroid and find a thread called Temperature Monitor or something of that nature.....he has a paid version in the Market but he also has a free version there in his first post....thats what I use
This is what you're looking for. It has the ability to put system temp in the notifications bar (in C or F). It's the same app referenced above by natediddy1120. Works fine.
I can think of three things off the top of my head that monitor temps. The one mentioned earlier, a widget called battery left or something, and an app called spare parts. I'm pretty sure the one mentioned in this post already is the only one that will display in F tho.
Keep in mind that all the Battery apps display the battery temp, not the core temp. Temp Monitor does display the core temp, or the closest thing to it.