Is it possible (and if so, how) to tell if a drive has JiffyDOS installed without opening it and looking for the chip?
If a drive has JiffyDos installed, it will say so in the first message it sends to the error channel on power-up. You should be able to read the error channel to determine if JiffyDos is present. Try running the following program in BASIC after powering the attached drive on to read the error channel. (assumes drive is set to device 8)
10 OPEN 15,8,15 20 INPUT# 15,A,B$,C,D 30 PRINT A,B$,C,D 40 CLOSE 15
The output will be the Error #, Error Name, Track, and Block (sector). You should see a message from JiffyDos in the Error Name if its installed.