The tolex is a good guideline to determine an A or B, but not definitive...as you can see, the picture posted shows a Version B with the old (Version A) tolex. I have a short head with the same tolex, also version B. I believe there was a short period of time in 1993 where they used the new Version B amp, but was still using the old tolex and SUS-4 cabinet.
Also, as mentioned, the early B's had the attached power cord, so that is also not a definitive indicator.
The effects loop is the way to tell. As mentioned:
Version A has two mono effects loops, one normal one switchable.
Version B has one effects loop with a mono send and stereo return. Version B also has a Satellite EQ jack (to send the EQ state to switch the EQ on a satellite amp), a Satellite Send jack (buffered output of one of the effects loop returns, as well as a Reverb jack, to switch the reverb on and off.
Note: some (or maybe all) Version A's also had a Satellite EQ jack, to activate the EQ on a satellite amp, but this jack was located under the chassis and not labelled.
Hope this helps. If someone posts a picture of the rear of a version A, to contrast with the posted picture of the Version B, the difference will be easily apparent.