Well, in the case of this particular single, it's not difficult to know which is the A side:
But there are many others from which, as the OP says, it's hard to tell which one is the A side if there's no such label. In my case, I found that plenty of those "side unmarked" singles have a number (different to the catalog #) for each side:
Consecutive numbers, as you can see. Don't know what they mean, but anyway they are enough side ID for me.