This resource is a little tongue in cheek, however it provides some sort of baseline for how hot something feels to the touch.
What does it score on the finger test:
0: Feels cool (cooler than if I touch my own arm)
1: Feels very slightly warm (about the same as touching my arm)
2: Feels warm, but not hot
3: Feels very warm, but I could leave my finger there indefinitely without any fear.
4: Feels hot. I can leave my finger there for a while, but I'd prefer not to.
5: Feels quite hot. I can leave my finger on it for a few seconds, but that's about all
6: Feels really hot. I can touch it, but I can't leave my finger resting on it.
7: Extremely hot. I can touch it, but I have to check my finger for burn marks afterwards
8: Superlatively hot. It feels really hot before I get my finger near it, and I don't want to leave flesh behind.
9: Glowingly hot. The device is smoking, in flames, or glowing with radiated heat. Fingers are well away.
10: House on fire. Not going in to check. Fingers dialling fire brigade.
4 or below is almost certainly OK. 5 or 6 are OK in some cases. 7 is generally only seen in devices obviously made to get hot. 8 or above are almost certainly signs of faults. (This is obviously a very rough guide)