I think the Pots will return 228 if not attached.
Second Pokey should be as per Pot4-7 on XL/XE since they're not normally attached.

IIRC, the method I put forward was: start a Pot scan, then read POT4 at $D204 and POT4 at $D214.
If you have two Pokeys installed, then one should be counting upwards, the other should remain static.

Another method which probably would be easier:

Set second Pokey's SKCTL to 3, then first Pokey's SKCTL to zero.
set Wait a little bit, then read RANDOM from $D21A.
Or it with same address once or twice. RANDOM should read zero if Pokey is in an init state.
Zero from second Pokey's address will mean that you have a mono machine since mappings repeat
each 16 bytes. Stereo machine, mappings normally repeat each 32 bytes for each Pokey. 