Here is a formula to calculate actual hashrate without relying on pool numbers, which can be inaccurate:
difficulty_1_shares * 65536 / time_in_seconds
If you are using cgminer difficulty 1 shares are easy to obtain from the API ("Difficulty Accepted" in "POOLS" data, you can also add "Difficulty Rejected"). Time in seconds is the time elapsed since cgminer started. Newer cgminer versions also have this in the API ("Device Elapsed" in "DEVS") but unfortunately 3.1.1 doesn't so you'll need to get that from somewhere else (e.g. ps in Linux).
There are plenty of examples out there on how to query the API. Here is a quick one for Linux (red things are the ones you type in, green are the important bits):
such@moon:~$ telnet 127.0.0.1 4028
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
{"command": "pools"}
{"STATUS":[{"STATUS":"S","When":1402719573,"Code":7,"Msg":"3 Pool(s)","Description":"cgminer 3.1.1"}],"POOLS":[{"POOL":0,"URL":"stratum+tcp://useast.wafflepool.com:3333","Status":"Alive","Priority":0,"Long Poll":"N","Getworks":52,"Accepted":23,"Rejected":0,"Discarded":101,"Stale":0,"Get Failures":0,"Remote Failures":0,"User":"richbeyondimagination","Last Share Time":1402719533,"Diff1 Shares":12288,"Proxy Type":"","Proxy":"","Difficulty Accepted":11776.00000000,"Difficulty Rejected":0.00000000,"Difficulty Stale":0.00000000,"Last Share Difficulty":512.00000000,"Has Stratum":true,"Stratum Active":true,"Stratum URL":"useast.wafflepool.com","Has GBT":false,"Best Share":8956}],"id":1}Connection closed by foreign host.
such@moon:~$ ps -eo pid,cmd,etime | grep cgm
7287 /such/mine/cgminer-zeus -S 15:53
such@moon:~$
EDIT: Lower pool difficulty will give you more accurate numbers, and obviously a longer time period will too.
