You could compare frequencies.
write your code so that one output pin is pulsed throughout your program, or better yet, try to take the pulses directly from the crystal itself.
Once you get a pulse, make yourself another oscillator. Both of these oscillators make an input of a digital PLL.
I think the simplest way to go with the PLL design is to connect one input to one input of an xor and xnor gate, and the other input to the other input of an xor and xnor gate.
connect the outputs of these gates to separate indicators. Two LED's with current limiting resistors will do.
The reason why I say use xnor and xor is because 1 LED will tell you when the frequency is the same and both oscillators are outputting different polarities, and the other LED will tell you when both oscillators are outputting the exact same signal.
both LED's cannot be on at the same time.