I had an attic space finished, and part of it involved routing existing 4" insulated flex-duct from the first-floor vent fans over and around joists. Now the fans appear to be fully obstructed.

Is there any trick to finding how far into a 25-foot run of flex-duct is an obstruction? Nobody I talked to had any ideas but there must be something better than ripping up floors and drywalls to expose the whole run?