I don't know any way to test DE filters, and ID clogged grids.

Was the clean filter pressure lower in the past? IF that is the case -- for example, if your filter used to show 12 psi when clean, and now shows 20 psi -- that would be a strong indication that your grids were clogged.

One way that can happen is if you acid clean grids without first cleaning them in detergent, to remove oils and organics.