For the road tires I've used, the speed they start squealing at gets lower after every lap. This is noticeable over several laps on the same turn. Also, it'll feel like you're slightly sliding while cornering as if there is a small layer of grease on the track or tires.
Depending on the tire compound and not having debris (chunks of tire, sand, dirt, fluids, etc) on the tire, it's also entirely possible to do very quick laps while they're overheating. The melting compound may provide better grip up to a point, but the tire will wear down quicker. If the track surface (or road to cool down area, paddock) isn't clean, debris will build up at different hotspots of the tire (as in the picture I posted) and that's when you'll start noticing grip/traction problems.
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