Here's a nice flowchart for determining what is killing you:
1) Does your anti-missile sensor pick it up?
1a) Yes: Does the missile size stay constant?
1a1) Yes - It's a missile of size x. Send in the cannon fodder.
1a2) No - It's a plasma torpedo. Prepare the rescue fleet.
1b) No: Does it hit your shields and/or armor?
1b1) No - It's a meson cannon. Do nothing, since nothing will stop it.
1b2) Yes: Does it affect your shield and electronics exclusively?
1b2a) Yes - It's a High Power Microwave. Prepare the popcorn.
1b2b) No: Does the shape of damage it does to your armor look like a funnel?
1b2b1) Yes - It's a laser. Attach it to a shark, send it back.
1b2b2) No: Does it fire multiple times per turn?
1b2b2a) No - It's a particle beam. Send it to CERN.
1b2b2b) Yes: Does it tickle?
1b2b2b1) No - It's a railgun. Laugh, then kill it.
1b2b2b2) Yes - It's a Gauss cannon. Nuke it from orbit.
In all other cases, it's another ship trying to occupy the same space-time coordinates as yours.
Alternatively, you can go to the Diplomacy (CTRL-F5) screen, look up the enemy class and if it's a beam weapon it'll tell you what type of weapon it is and the observed range. Too bad the same doesn't happen for missiles.
Edit: Forgot the Plasma Carronade, but I'm out of ideas on where that goes, so I'll just refer to the spoilered portion.