It should be possible to do that by monitoring estrogen and progesterone concentrations in urine or feces.  Are bats induced or spontaneous ovulators? 

If you know the reproductive cycle of the species, it would be necessary to make capture-recapture of adult females before, during, and after the breeding season. So that you can practice a vaginal smear to record each one of the particular cell types at each stage of the estrous cycle.

You could also use such smears (noninvasive method) and / or blood, in assessing sex steroid hormones, which will allow you to correlate stages of the vaginal epithelium cycle with the endocrinology of the species.

For more information about this I recommend you read:

Chavan and Dhamani. 2012. Hormonal profile during ovarian cycle in the Emballonurid female bat Taphozous Kachhensis (Dobson). Academia Arena, 4(6): 30-36.

This paper can help you:

Anat Rec (Hoboken). 2011 Mar;294(3):506-19. Ovulation, fertilization, and early embryonic development in the menstruating fruit bat, Carollia perspicillata.

Rasweiler JJ 4th1, Badwaik NK, Mechineni KV.

I think this is possible through urine and /or fecal matter analysis. one more way if time and FD permission permits you marked few females and recaptured through mist net and get blood or urine to do the analysis.  if you willing to do through non-invasive technique urine or fecal is the best way to collect hormone data...