There are a couple of ways:
lscpu or more precise
lscpu | grep "MHz".
This will give you the general MHz for the CPU.
cat /proc/cpuinfo or more precise
cat /proc/cpuinfo | grep "MHz".
This will give you the individual MHz for each CPU Core. So if you have an Core 2 Duo, AMD Bulldozer, Core i7, etc.. it will show the MHz for each core.
lshw -c cpu or more precise verion:
lshw -c cpu | grep capacity
Will give you the general MHz. Same as
sudo dmidecode -t processor or more precise:
sudo dmidecode -t processor | grep "Speed" Will not only give you a MHz in use but also the Maximum you can push / overclock your CPU to.
Out of all of this,
dmidecode provide the best information out of your CPU.
You can also target the current MHz detected by the kernel by querying the log files:
cat /var/log/dmesg | grep "MHz processor" - For the current detected MHz speed
cat /var/log/kern.log | grep "MHz processor" - For the current and past detected MHz speeds. Will not work in some cases, that is why I posted the
dmesg one first.
And that's all I can remember from the top of my head. I am fairly certain there are other ways, just don't remember right now. Of course, talking about terminal ways.