Don't know of any specific commands but you can always take a peek inside
/proc/cpuinfo.
edit:
cat /proc/cpuinfo
[/edit]
If you have an x86 processor then the cpuid assembly instruction will get you a lot of information about the cpu.
This will give you some idea of the speed but not an absolute value.
Hopefully someone with more knowledge/experience than my next-to-none will be able to offer you a more useful suggestion.