How to Get The Type of Fluid in a Tank Block

How do I get the type of fluid from a block with fluid storage in a procedure? You can test to fill a certain type of fluid, but not test to drain a certain type of fluid. If I want a tank to be able to hold multiple liquids, and return that liquid to allow it to do different things based on the liquid in the tank, I don't know how to do that, or if it is possible.