Here's what one bladder manufacturer says. You might look for the manufacturer of the ones you have.
13.) Leak Testing: Prior to use, all tank and container type products should be leak tested. This procedure can generally be done with 1/8 psi air pressure and a standard bubble-type leak detector solution. Follow the User
Manual procedure or contact ATL for help. Always rinse and dry the surface after testing.
Note only 1/8 psi as they are not built to handle pressure above just normal expansion of fuel.