The standard deviation is one common calculation that does this. Assessing the possibility of systematic behavior is somewhat more subtle. In general is a measure of how much a single measurement fluctuates from the mean. In this run you have made fifty presumed identical measurements. A better estimate of how well you have really determined the mean is to calculate the standard deviation of the mean .
Systematic errors can sometimes drift over time.
In the best-case scenario they drift up and down so that they hopefully average out to zero.