Is there a way, and if so, what's the best practice for checking a mushroom to verify it is clear of worms?
According to the CRC
"Mushrooms – Fresh mushrooms must be rinsed and a cursory inspection is needed. Portabella mushrooms should have the brown under part removed. All dried and frozen without any added kosher sensitive ingredients are acceptable. Canned or jarred mushrooms must have a reliable hashgacha." Source: http://www.crcweb.org/fruit%20&%20veg%20guide.php
According to the Star-K
"H. Mushrooms (all types) 1) Wash well, especially the fan area under the crown. 2) No further checking necessary." Source: http://star-k.org/bugcheckingquickreference.pdf
According to the OU
Type of insect: Small white or red worms
Location of insect: Imbedded in underpart or inner sections of the mushroom
Method of Inspection:
Shitake and Button: Wash thoroughly.
Oyster: Break apart in several places, especially the base; examine inner sections. If insects are found, discard mushroom.
Portobello: Remove stem; examine detached cap. Remove entire brown fanlike underpart. Wash thoroughly." Source: https://www.ou.org/jewish_action/12/2006/kosherkopy_debugging_your_home/