For ASCII characters, something like this would work:
cmp eax,'A'
setnc bl ; bl = (eax >= 'A') ? 1 : 0
cmp eax,'Z'+1
setc bh ; bh = (eax <= 'Z') ? 1 : 0
and bl,bh ; bl = (eax >= 'A' && eax <= 'Z')
; bl now contains 1 if eax contains an uppercase letter, and 0 otherwise