You can actually tell the difference by looking at how much space the grooves take up on the record. The instrumental, being at least 30 seconds shorter than the vocal, takes up noticeably less space. In other words, the runout space is larger on the instrumental. So Pete is nearly correct, you do need both versions side by side to tell the difference without playing the record. Having said that, there is also a way of telling the difference between the two by looking at the numbers in the runout grooves.