I see that MacWorks Plus installs a ROM image which identifies itself using the same version word as the Mac Plus ROM: $0075
I have code that checks the low byte of this word and if the high-bit is set, the code branches for Lisa/MacWorks. This code does not detect MacWorks Plus.
What should I do to detect the presence of MacWorks Plus?
MacWorks Plus / MacWorks Plus II typically masquerade as a Mac Plus because there was some software that would refuse to run on an X/Lisa on the assumption that MacWorks could not run it properly (which was true of MacWorks XL, but no longer true with MacWorks Plus ... more or less the reason for MacWorks Plus).
For its own purposes, MacWorks Plus code variants (eg. installers, desk accessories) check the long word at $400040 to find:
MW+ : 'MACW'
MW+2: 'MW+2'
There are also MW+/II version locations which contain p-strings:
MW+: $400088
MW+2: $400080
Since the version string isn't entirely predictable or reliable due to patches, it would be best to avoid making general decisions based on the version string.
As usual, you may choose to trust, but verify. HTH.