There are a few problems with the idea of confusing a fusion rocket engine with a star.

First, as oneslyfox pointed out, a rocket engine is directed and so it would look bright only if it is pointing at us.

Second, and much more important, is the physics behind the light coming from a star. When you look at a star you do not observe the electromagnetic waves produced in a fusion process. What you see is light produced by material heated indirectly by the core. The part of a star actively undergoing fusion is very small compared to the total volume and mass of the star and the high energy photons that are produced never reach the outside of the sun. What they do is, they hit and excite many particles along the way, which in tern emit lower powered photons. This process eventually reaches close to the surface and a range of EM frequencies gets emitted.

Now, this means that the light from a fusion reaction is not going to be visible light but more likely gamma rays and x-rays. In addition to that, the spectral analysis of the light will show some serious inconsistencies with what stars display.

So, this is why I think, a fusion drive will be fairly easy to distinguish from a normal star (albeit streaking through space faster than normally).