How can I figure out where squirrels are entering my attic?
There may be many points, and they may not be what you think. First off, squirrels can squish themselves down and fit through a hole much smaller than you'd think they could. Don't rule any holes out because they're too small (well, other than say the vents in your soffit). 

Check soffits, vents, corners, anywhere wiring or pipes enter (anywhere in the house). 

It's possible they're coming in around the foundation, or through your garage, and then climbing up walls or even on the inside of walls. Look for any holes, including under decks, stairs, etc. 

In the winter, seeing their tracks in the snow can point you in the right direction (but the sooner you can get rid of them the better so don't wait until winter).