The first and most obvious of those is a tear or hole in the fabric. Since the entire purpose of the filter grid is to hold the Diatomaceous Earth in the swimming pool filter, then a hole, allowing the D.E. to return to your pool, is no good for you. (Note, if you are seeing DE getting back into the pool then a hole in the grid or crack in the manifold is the most common source.)
A second indicator a grid needs to be replaced is broken plastic fins with in the grid. With as much trouble as it can be to take the filter a part, clean it, then put it back together. The last thing you want to do is do it again 1 week later. Filter grids that broken plastic inside of them are on borrowed time and soon will be generating holes.
Remember a good rule of thumb is that if 4 or more grids are broken, replace them all because the others are not far behind. And as always Swim Clear is here for all your swimming pool help needs such as filter grids if you need them.