Front disk brake(s) sticking- where to start toubleshooting?

First thing to do is determine if it's hydraulic. "Get the problem to show" and be prepared in advance. That is, have tools, rags, cardboard to lay on, a handy jack, etc, plan it out. When the pedal gives you the "feel" you describe, immediately go out and get some rags in there and crack the tubing fittings at the master and see if there is pressure holding.

If it still sticks and the above did not "show," then next time crack the bleeder on the caliper. If one side seems to heat worse, crack that side.

If you have the time, you can jack up that wheel and confirm "you gained" be rotating the tire and feeling for "looser" after cracking the bleeder, that is, if there's any question.