Really, it doesn't matter if you filed with FSS or DUAT/DUATS. They all go to the same place.
That being said, check the AFD to see if the airport has a GCO or an RCO to a local APP/DEP facility, an ARTCC or a FSS. If so, you can simply use the radio to retrieve your clearance.
If none of the above are available, you can call Flight Service on the phone and have them relay the clearance to you, with a release time and a clearance void time. So, make sure you're ready to start up and launch!
If the weather is good enough for you to depart VFR and the airspace permits, you can also depart and simply contact the ATC facility in the air for your clearance. NOTE: If you use this method, make sure that you can maintain VFR conditions AND maintain your own terrain an obstruction clearance long enough for you to establish communication with ATC, be positively identified on radar, and receive a clearance.