You can easily tell. The rubber is just that, rubbery. Press on it and see if it gives. Most of the time the contact glue used is pretty thin and doesn't hold well, so the rubber will stretch a little. Fiberglass won't budge. Also, if the roof is chalk like, leaving a lot of chalk on your hand after rubbing your hand over the roof, it's most likely rubber. I've been doing a lot of research because my FR/TT has a rubber roof. I'm about to re-coat it with liquid EPDM (liquid rubber). From everything I've read about rubber roofs should not be cleaned. Original rubber roofs naturally chalk, pretty much being self cleaning. Cleaning them just removes more of the chalk, making the need to either replace the entire roof with new rubber or re-coat with liquid rubber take place that much sooner. If you're still unsure, you can remove one of the small vents on the roof. You'll see the roof material cut for the vent. The pieces there will obviously be rubber or fiberglass. Or, the best suggestion yet, see if you can find the specs for your trailer online.