Hi Kevin,
I think it's impossible to messure crystal frequency by the micro.
there are two points:
at first the micro (the standard 8052) cannot measure any frequency above its crystal frequency/24 ( i.e. XTAl:12 MHz-> 500 kHz)
so it is impossible to get any frequency higher than that.
second point is:
every time measurement you do is based on the crystal frequency.
i.e. if you use an frequency divider (/16) and you count the periods
of the divider output signal you must do this in a time window.
but this time window always based on the crystal frequency!
i.e. one timer runs free and the other is used to count the divider
clocks and counting is enabled befor starting the timer and disabled
by the timer overflow interrupt routine.
then it doesn't matter what crystal you use because number of clocks
counted will always be the same.

In your case you will need an external time reference on which your
measurement must be based.
if you need the frequency to generate correct baud rate for RS-232
think about using the 22.1184 MHz crystal and use auto baud detection
to get the baud rate(see code libary for that)

best regards
Christian