How do we know pulsars have two beams? Pulsars are neutron stars the emit a beam of electromagnetic radiation that is not aligned with their rotation axis. So we can only see a neutron star as a pulsar if the beam crosses Earth's path. 



The common representation is that there are two beams, one on each side, but unless the magnetic axis and the rotation axis are at 90°, we can only ever see one of the two beams. How do we know that there are two beams and not one, or four, or any other number?
Several pulsars have double-peaked profiles: you get two peaks per rotation, with one peak stronger than the other. An example of this is the Crab Pulsar. This suggests that at least some pulsars have two beams. Remember that the gravitational bending of light in the vicinity of the neutron star makes it possible to see a larger fraction of the surface, which increases the chance of both poles being visible.

The single-peaked light curves likely represent the case where the second pole is not visible. Of course, if you wanted to play Devil's advocate, you could argue that there might be single-beam pulsars lurking in that sample... you'd probably need to come up with some kind of physical mechanism by which such an object could exist, given the physical theories that have so far been used to explain pulsars predict two beams.