What do you need to do?
If you find a fungus with larvae in it, retain it in an open container - do not place in a plastic bag as CO2 will build up and kill the larvae. When you get it home, place it on top of some sterilised coir (heated and dried in the oven to kill existing larvae) or similar material that will soak up the decayed fungus and provide a pupation site for the larvae. Cover the container with a fine gauze and place in a position where you are likely to regularly inspect it.
In due course, fungus gnats and other fungus-feeding flies (e.g. Platypezidae, Helomyzidae and Drosophilidae) should emerge. These can be removed from the container and stored for later identification. If you don't have access to ethyl acetate, which is the normal killing agent used by entomologists, then the freezer works just as well.
Keep a record of where your fungus was found, what species it was (if not to species then to genus) and when the flies emerged from it. Later in the winter these flies can be passed on to the fungus gnat recording scheme and will help to fill in gaps on the maps but, more importantly, they might add useful ecological information on the host fungi used by different species.
Try watching fungi for the flies that visit them, and then catch the flies up for passing on to the recording scheme (care needed not to inhale fungal spores if you use a pooter). Armillaria is especially good and can yield a lot of species if one takes a diligent approach to watching and collecting.
The majority of fungus gnats require dissection and cannot be identified from photography, but there is an exception in the Platypezidae, some of which can be identified from photographs.