First thing I would check is that the buttons all work ok. I brought my asr new and had to replace the whole top panel after 2 years because the buttons wear out. Especially the cursor keys up/down/left/right. Secondly if its the version with the older gas plasma display, make sure it works properly with no bits of letters or numbers missing etc. If its the newer lcd type display then dont worry but just know that this version of the keyboard is slower in operation than the earlier models due to the fact you have to use the latest o.s and cannot use o.s 2.5 which is much quicker.
Give both end keys (bottom C and top C) some firm presses to make sure they are not broken or worn out.
Other than that, if it goes it goes. One point that happened with mine is that the SCSI was retrofitted - After it was, when using 44.1 khz sampling rate there was (intermittantly) lots of nasty digital noise and feedback due to incompatibility with the SCSI board version and the ASR mainboard version. I've only ever known of this on my own keyboard but it can happen - ENSONIQ TOLD ME THEMSELVES when they were still around!! unfortunately they went bust before I got mine sorted. Its worth just mentioning to the seller and asking if 44.1 works ok.
Basically, if it boots up and the screen seems fine and the buttons seem fine then it probably is fine. Check the floppy works ok too.
Go out and get it. It's the best sampling keyboard ever made and there is nothing it cannot do with samples. Good luck.