Ok first of all there isn't really one simple rule for knowing all of this but once you have read through all of this below you should know what is backside and what is frontside.

Grinds and Slides: For a F/S Grind or slide of any kind you have to come up to your target facing it - Your toes on your board should be facing the object, you should be able to see it in front of you not behind you. If that is what is going on then you know you are doing a F/S grind or slide of some sort, for a B/S you have to be the exact opposite. Your back should be facing your target - you should have to turn your head and look over your shoulder to see it and the heels of your feet should be pointing at the target, then you know that it is Backside. Many people get confused on this because the think the easier one should be called a frontside and harder a backside. This is kind of correct if you think like most people on a 50-50, that F/S is easier then B/S, or that you learned F/S first. Then when you are talking about board slides a B/S is easier and most people call it a F/S. That is wrong, go by the rules above...

Spins and Flatland Tricks: Ok this is even more confusing then Grinds and Slides, but once again, once you understand this it is very simple. For tricks where you spin, like 180's, 360's and so on, a F/S would be when you are turning and for the first 90-180 your can see what is going on in front of you. For example if you are regular and you are doing a normal (Not fakie or nollie or w/e just normal) F/S 180 your shoulders turn to the left (If we were looking at your from a bird's eye view). For a goofy footed rider your shoulders would be turning to the right (again from a bird's eye view). For a B/S spin you should not be able to see what is going on in front of you for the first 90 degrees or more. Another rule for this is for a F/S your toes are facing outward the whole time (In front of you) and for a B/S your heels are facing outward, in front of you when you are spinning.

Now read that through a couple times until you understand it you have to understand spins before you understand flip tricks to actually get something out of this (The flat ground tricks part) Ok, so you got the spins now the flat ground stuff isn't much harder to understand, all you have to do is thing of you as being your board when it is spinning. So let's look at a 360 Flip (A B/S 360 pop-shove-it with a kickflip) I will explain how I got all of that in the (  ) things right now. You know that a B/s 360 is when your heels and back are facing outwards/forwards for the first 90-180 degrees or so and when you cannot see what is in front of you for at least the first 90 degrees of the turn. So think of yourself as being your board when doing a 360 flip (don't worry about the kickflip part, that's just extra) if you were the board all of the rules would apply to a B/S 360 (I am not listing them again...) So you know that a 360 flip is a B/S 360 Pop-Shove-It and then the extra that is left is the kickflip.

Review: Ok so now you know all you need to know about B/S and frontside, the same rules apply to when doing fakie, switch and nollie. So If you can see what is in front of you for the first 90-180 degrees and you are doing a Fakie 180 you know you are doing a Fakie F/S 180, or a F/S Half-Cab. If you are doing a nollie 360 and for the first 90 degrees your back is facing the direction you are going and you cannot see what is in front of you then you know you are doing a Nollie B/S 360. 

The same rules apply to 360's and 180's and any other spins, the only thing you need to see to tell if it is a B/S or an F/S for a spin is the first 90-180 degrees of the spin. If you really know your stuff you can just tell after the first noticeable turn in the air. Whether that is 1 degree or 15 doesn't matter, as long as you know what is what.

Well I hoped this helped!

Good Luck!