curious proyect xD may be its not the best idea but i could be a good way xD
for this you need 2 pics use de cheapest
the first one will work with a refference xtal value (like 4Mhz)
the first pic will wait until the second one sends a signal like 1 or 0 from porta,4
when u introduce the xtal on the second pic (the one u want to know the value) it will send the signal to the first one and both will start a counting subroutin. at some point just compare those numbers.
for example:
pic 1 = 200
pic 2 = 400
(pic2/pic1)*osc reference = (400/200)*4mhz = 8mhz
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you also can use a 555 istead the first pic, (i dont know how to say this in english)use configuracion astable with a low pulse width of 1 second, make a 1 second subroutin for the pic too (supposing its a 4mhz xtal), the a counter ic for each one, at some point, move counting value for each one to the pic, compare them and then show the result (something like in the first option xD)