If you have <x> <y> on the stack, you can verify that y=x+1 with:
2DUP
1 SWAP OVER CAT TOALTSTACK CAT
1 ADD FROMALTSTACK EQUALVERIFY
The process is just to append a 0x01 byte to both suffixes, to ensure neither value is treated as negative or non-minimal (0x00 or 0x80 or a multibyte string ending in 0x00 or 0x0080), then increment, then check. The suffix can be one, two or three bytes this way, so this only doesn’t work if the smaller hash ends with FFFFFF, but that’s a one-in-16-million chance, instead of 1-in-2 or 255-in-256.