Lay your jacket on a table or flat surface.
Spray the jacket with water.
Observe the water.
If the water sinks in and absorbs into the material, your jacket is "wetting out" and needs to have the durable water repellent finish reapplied.
If the water beads up, staying on the outside of the garment like dew drops, your jacket does not need to have the durable water repellent finish reapplied.
Water droplets should roll right off of the jacket when you pick it up.