The backs of the slides are pretty easy to spot because they will have virtually nothing on them except a date or a number. Some people will write their slide information on the back of the slide because they will be sitting in back of the projector and maybe putting the slides in one by one and then can read the back of the slide as it is put into the projector. There are exceptions to this rule, as you can see below but, if you see a date or a number, it is most certainly a "back" and not a front.