I have a pair of chest waders that are only a couple years old but have a few compromised points as my last couple trips ended with soggy feet and wet legs. I have purchased Aquaseal and placed all around the stitching but water is still getting in.
I am not sure how to preform a systematic test to see where the water is coming in.
I only know it is in both boot areas and then higher up from the look of my sweatpants after a fishing session.
I realize getting a boat or buying another pair would fix this problem but I have been unable to convince the wife that a boat would also benefit her.
To find the leaks turn the waders inside out and load up a small spray bottle with hydrogen peroxide and just mist that over the waders. The peroxide will foam and turn white where there is small holes and then you can repair them with the aquaseal. For seams I do multiple thin thin thin thin coats of aquaseal (4-6) so that you maintain as much flexibility in the waders as possible. Eventually though they become more aquaseal than wader and it's time to replace them. Patagonia usually has a really good sale every Jan/Feb where they have all their previous years waders online at a big discount.
I've also turned them inside out and filled them with water (but you need to find a high place, like a tree branch or jungle gym or something to hang them from),