The maximum distance between any pair of points in a set of points is called the diameter of the set.
Here is one efficient algorithm, based on the convex hull, for solving this problem:
- http://www.tcs.fudan.edu.cn/rudolf/Courses/Algorithms/Alg_ss_07w/Webprojects/Qinbo_diameter/2d_alg.htm
- http://cgm.cs.mcgill.ca/~athens/cs507/Projects/2000/MS/diameter/node2.html
Since you probably don't care about exactness here, it would be easier to just find the minimum and maximum latitude and longitude over all of the points, and test whether a side of the box defined by these extrema is larger than some threshold. This works assuming you don't care about users near the north or south pole.