There are two current threads going simultaneously in this section on rain jackets. I'm not going to try to merge them (with my luck, it would result in chaos), but I suggest you read them both.

One possible test is to stand in the shower (cool water) for about 30 minutes with the rain gear on. That doesn't cover the situation where the DWR wears out after 6-12 months and, despite several efforts per the manufacturer's instructions, can't be renewed, but at least it lets you know if the jacket will keep you dry the first few times you wear it. Hopefully the jacket will be returnable (dry it carefully) if it fails the shower test shortly after purchase.