A dry cigar tells on itself before you light it. The wrapper feels brittle or papery in your fingers; gentle pressure produces a faint crinkling sound rather than the soft springy give of a properly humidified cigar. The wrapper may show fine cracks running with the grain, especially near the head where it is rolled tightest.
If you light it anyway, five symptoms follow. The burn runs fast and hot because dehydrated tobacco combusts more readily. The smoke is harsh in the first inch, particularly on the retrohale. The draw feels loose, almost airy, because the filler has shrunk and lost its packing density. The ash is light grey and flakes off in small pieces rather than holding in long sections. The cigar may go out repeatedly between puffs as the burn loses its rhythm.
The fix is rehydration, but slow rehydration. Park the cigar at sixty-five percent humidity for several weeks, not several days. Do not put a dry cigar at seventy-two percent and expect it to recover overnight; the wrapper will absorb moisture faster than the filler, and you will end up with a wet wrapper around a still-dry centre that cracks the next time you handle it.
The full diagnostic, including symptoms for too-wet and too-fresh cigars, is in How to Rest a Cigar.