Testing if a point-pattern is on a regular grid

Dear list,

I'm looking for an easy way to test if a point pattern (I got a data
frame of x and y coordinates) is on a regular grid or not.

I'm aware of the gridded() function in sp(), but this function is
devoted to assign rather than to test, as far as I know. I'm pretty
sure there's an easy way to do that though....

Any suggestions ?

Thanks,

Pierre