If you do a 'show controller' on a physical serial interface, included among the return data should be a line indicating the cable type and received clockrate. Here is an example:

Router#show controller Serial0/0

M4T: show controller:

PAS unit 0, subunit 0, f/w version 1-45, rev ID 0x2800001, version 3

idb = 0x6245AC6C, ds = 0x6245C974, ssb=0x6245CC4C

Clock mux=0x0, ucmd_ctrl=0xC, port_status=0x74

Serial config=0x8, line config=0x200

maxdgram=1608, bufpool=48Kb, 31 particles

DCD=up DSR=up DTR=up RTS=up CTS=up

line state: up

cable type : V.35 DTE cable, received clockrate 56000

[ more output cut ]