Gums, there are very specific AMM procedures for aircraft storage - somewhat tailored for the expected length of storage - although I'm not familiar with the specifics. Aircraft go out off and back into service on a somewhat regular basis (Boeing sometimes does 'build ahead' - operators generally want to take delivery right before the summer peak season, but monthly production stays consistent - so aircraft go through production flight test then right into storage for several months prior to delivery), although not often on the scale we're currently experiencing. I think the only previous large scale experience was after 9/11 (in early 2002, United was taking delivery of new 777s (sans interiors) and flying them to the desert and parking them). This one is also unique that an entire model of aircraft has been affected for an extended time.
As for a check flight - again I'm not familiar with the specifics - but I'd expect that the positioning flight from the storage location to the return to service location would be used as the check flight (which is when the subject IFSD occurred).