Your miner software will spit out your hash rate every second or so. The pool that you are connected to will also estimate your hash rate based on the number of shares you submit to them in a time period.
The rate is how many sha256 hashes you are achieving every second so 250 Mh/s is 250 million per second.
Hope that helps
Topic: What is Hash rate and how is it measured? (Read 908 times)
How do you measure hash rate?All i know it is how fast you can mine... 
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