What if you don't recognise an aircraft ATC is telling you about?
To add to Dave's answer, if traffic is pointed out to you which you are unfamiliar with, ask for clarification. ATC or FIS will have a good understanding of aircraft types and speeds, especially in radar environments the speed will be available as groundspeed (GS).


  FIS: DELFF, traffic information, 3 o'clock, 4 miles, same altitude, Mooney Bravo, passing right to left.
  PIC: DELFF, unfamiliar with aircraft type.
  FIS: DELFF, traffic is a low wing aircraft, 100kt groundspeed