If it's a treadless tire, then calipers may be the only way to know for certain how thin the rubber has gotten. Or to perhaps inspect closely with a bright light and magnifying lens (I have admittedly old eyes, maybe you don't need the lens) to look for evidence of fibers showing through the rubber. I agree that waiting for an increase in flats is pretty inconvenient. I prefer to replace too early rather than too late.
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