It is usually checked by means of various check standards. The true value might be a solution of known concentration, such as a pH standard buffer. This is because generally a check sample is measured more than once, and the average of the results is compared to the true value. For instance, let's say your accuracy limit for Secchi readings is plus or minus 6 percent. The volunteer's result is acceptable.
Precision describes how close values are to each other.
The smaller the CV, the better the precision. Let's say your lake monitoring volunteers test dissolved oxygen at several depths, and on each sampling date they collect and analyze one duplicate sample from a depth randomly selected by you. 8.5 0.14 1.6 Vol. 2 5.6 7.2 6.4 1.13 17.7 Volunteer 2 exceeded the precision limit for the day, so all dissolved oxygen data from this volunteer on this day should not be used. When samples are analyzed in a lab, it's important to know both total and lab precision.