Checking plays at the LOS?

Simple way to do it:

If you use a number in your cadence...for instance "Red 37, Red 37!" then simply use "1" as a number to represent the first play called in the huddle and "2" to represent the second play called in the huddle. So if you called a Pro Right Sweep and Counter, you can say "Red 1, Red 1" and that would indicate Sweep. "Red 2, Red 2" indicates Counter.

If not, then just have the QB yell "1" or "2" after he's read the defense but before he starts his cadence.

It's not as if the defense will know what the plays he called in the huddle were, so it shouldn't matter if it is obvious or not.