Sontaya,
Jay is correct in his description of using T0 and T1 to determine an external frequency, however you need to be aware of the limitations of the particular chip you are using. While Philips makes the greatest number of variations of 8051s (excellent choices I might add) I use many different vendor's chips based on circumstances not necessarily technical. If for example you are using the Intel 8051 you will not be able to count a frequency of 1MHz with T0 as a counter. The external input to the counter is sampled every machine cycle and is updated when a high to low transition is detected from one machine cycle to the next. This means that the maximum count rate is 1/24 of the oscillator frequency. So at 12MHz the maximum input frequency detectable by T0 is 500kHz. If you want 1MHz there are other schemes to use.
Allen