How to tell if preserved chillis are still good?
Alcohol is a disinfectant, so any bacteria sitting around for a month in vodka have been thoroughly killed...

The only thing to worry about is that if some of the chillies were not completely submerged all of the time, you might have a slight problem.  The symptoms to look for is discolouring: look for brown / black spots / extremities.

If not: no worries: they're still good.