PBP 2.50/2.60/PBP3
Code:
ASM
    if ((R1-R0) == 4)
        error "Compiled with PBPL"
    endif
ENDASM
PBP3 only (faster)
Code:
#IF __LONG__ = 1
    #ERROR "Compiled with PBPL"
#ENDIF
The second one is faster because PBP picks up the error.
The first one has to go through the assembly process before you get the error.