I guess hand sanitizer must have a shelf life, which must be available from the supplier on the data sheet perhaps.
Maybe you just need to ensure that you have the data sheet and a simple system for ensuring you are not using out of date hand sanitizer.
You could go the whole hog and swab employees hands prior to and after using all of the different hand sanitzers you have on site. And compare results to validate their ongoing effectiveness, but it might be a little difficult to operate such a system.
If you have a regular swabbing regime in place perhaps you could actually do some of the above, which would demonstrate the effectiveness of the hand sanitizer.
Just a few ideas.