This is a very common topic amongst most beginners and even some intermediates and I think someone (snowboard city) should clear this all up so that is what I am going to do.

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 rails a B/S boardslide is when you are on the rail facing downhill, or with your back facing uphill. This way seem weird, but it makes sence. For example, lets say your doing a handrail, you cant come straight on the rail, so you have to come from either side. A B/S boardslide is when you are next to the rail, your back is facing the rail, and then when you get onto the rail, you are facing downhill. A F/S boardslide is the opposite, with you facing the rail, therefore called frontside. And then when you get onto the rail, you are facing uphill. 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 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.

Review: Now you know how it works. The rule that applys to 360's and 180's and any other spins, all rails, and everything you do is that 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!