The test for acetobacter is simple: smell whatever's coming out of the airlock on your fermenter. If it smells like vinegar, you've got an infection :).
For a fermentation in progress, there's only a couple of options. You could pasteurize the whole batch, which would kill off bacteria as well as the yeast, so you'd have to repitch your yeast.
Another possibility is to dissolve some campden tablets into your cider. If you pitched a quality, healthy yeast, they won't be affected by the increaed sulfite concentration but it will inhibit bacteria and wild yeast growth. If you take this route and find fermentation stops, just give it some time and repitchm, as the sulfites from the campden will dissipate with time. Campden tabs are used all the time in wine and cider making to prevent infection without having to boil/pasteurize.