The best way to be certain that you have adequate washing is to test for residual fixer.
The way I wash prints is to create a make do cascading washer. I use 4 16x20 trays in a stand and a Kodak tray syphon on the top tray. The over running water splls from the top tray to the next etc. I wash 4 8x10 prints in each tray that I separate with 35mm wire reels. The prints are induced into the bottom tray and are moved upward when 1/4 of the time has passed. I have tested this method for residual fixer...kits are available from a number of sources and easy to use.. and have an established time to meet my needs for the regimen that I use.