How to check if light is "Power Through Fixture"?
Actually, the thing  you're referring to is called a switch loop. 

You can find out if you have a switch loop by opening up the switch, pop off hte cover plate and pull the switch out a bit.  

If you see one cable coming into the box with two wires (white and black), and both go to the switch itself (as well as ground if present), that is a switch loop.  

If you see one cable, but it has black, white and red wires -- and black and red go to the switch and white is capped off and connected to nothing -- that is also a switch loop, however it complies with current code, and that white wire is the neutral wire the smart switch is looking for. 

If you see one cable with black-white-red, and all three go to the switch, see "3-way switches" below. 

If you see two cables coming into the box, and the whites from each cable are wire-nutted or spliced to each other in the back of the box... and one wire from each cable goes to the switch (aside from grounds)... that is power at the switch.  That configuration is friendly to the use of smart switches.  The white wires bundled together are the neutral, and they are not "spare", they are in-use as part of the circuit. The smart switch's neutral can be added to these. 



If you see three wires (other than ground) going to 3-4 different terminals on the switch, and 1-2 of those terminal screws are black...  that's a different thing, that's a 3-way switch. That is more complicated.  But as far as smart switches, you have more options if you have that nice 3-way cable with its red wire.