On synchronous serial ports, one side of the connection is the DCE side, the other side is DTE. The clock is supplied by the DCE side for both directions of communication. Normally, this is the responsibility of the communications carrier. (as in "the phone company"). If you get a crossover cable to connect two Cisco routers together, one end will be marked DCE. On most cisco routers, you can supply a clock, which is needed to connect to another router instead of connecting to the telecommunications carrier.
To see what the clock rate was set to, enter (for serial 0) :
show controllers serial 0
the first two lines will be something like:
HD unit 0, idb = 0x2B507C, driver structure at 0x2BC640 buffer size 1524 HD unit 0, No cable, clockrate 64000
If you are the DTE end of a serial cable, you cannot tell what the rate is. In theory, it would not need to be consistant, i.e. each data bit could be a different time relative to the previous, ie, one byte could be set at 32000 bytes per second, the next half byte at 64000, the next half byte at 16000, etc.