How can you tell a fs trick from a bs?

If it helps at all FS means Front side and BS means Back side

If its grinding then BS is if you are approaching the grind with your back facing it. If its FS then its in front of you

If its rotation FS would be if you did a 180 degree spin spinning into your forward shoulder (Goofy = Spinning Clockwide / Regular = Spinning anti clockwide) If its a BS rotation you will start by spinning into your back shoulder. (Goofy = Spinning anti clockwise / Regulary = Spinning clockwise)

Then theres things like pop shuvits and normal shuvits. A FS pop shuvit would be you popping the board in front of you and a BS would be you popping the board behind you.

Grabs: Simple really FS grabbing the side of the board by your toes, BS grabbing the board on the side by your heels.

Inverts: Same as grinds but on the lip of a halfpipe.

Theres probably more but there all the ones i can think of.