You can look it up online - take the serial number and go to http://www.rolleiclub.com/cameras/tl..._numbers.shtml . There's a pretty comprehensive chart there of which model a given camera is based on serial #. To figure out the year, this is as good a starting point as any: http://www.rolleirepairs.com/models.htm .

That looks like a Rolleiflex 3.5, with no meter. That puts it somewhere between 1950-54. If that's not what it is, then it could be later than that, but your best starting point is the serial #. If you don't know where to find it, it should be on top of the body in front of the focusing hood, between the GBP and DBGM inscriptions.