As per the specification. If you power the raspberry pi from the USB port from PC,It will run at half speed of what it can achieve. But when you power it from the external source then it runs at full speed of 700 MHz. So I want to ask how can we see the clock speed in real time ?
There is a good summary of the CPU frequency scheduling interface for Linux. In short, you can run the following command to get the current frequency of your CPU:
sudo cat /sys/devices/system/cpu/0/cpufreq/cpuinfo_cur_freq
Update: With Raspian based on Debian 8.0 you can find it here:
pi@raspberrypi $ sudo find /sys -name '*cpuinfo_cur_freq*' /sys/devices/system/cpu/cpufreq/policy0/cpuinfo_cur_freq