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Although many will say there is none safe, I'd first look at the date stamp on each (numerical week/year) and get rid of them if they're more than 3 years old. Other than that, tires do get hot with road temperature, being forced to flex with lots of turns (that's why F1, Indy and other road racing cars zig zag so much when going slow on fresh tires), and when they start breaking down. I've been told that each is a little different according to their structure and tread design, and having the gauge on the stem puts it out away, but anything over 200 - 250 F means you've a problem. Below 150 on a warm day shouldn't be anything to worry about (but then you've those Marathons!).
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