Can bats echolocation discern what insects they are preying on?
Yes, some bat species can distinguish the species of insects by echolocation. One of the better studied species is the greater horseshoe bat (Rhinolophus ferrumequinum).

Echolocating greater horseshoe bats (Rhinolophus ferrumequinum) detect insects by concentrating on the characteristic amplitude- and frequency modulation pattern fluttering insects impose on the returning echoes. This study shows that horseshoe bats can also further analyse insect echoes and thus recognize and categorize the kind of insect they are echolocating.
von der Emde, G. &amp; Schnitzler, HU. J Comp Physiol A (1990) 167: 423. https://doi.org/10.1007/BF00192577

Further reading:

Hans-Ulrich Schnitzler, Elisabeth K. V. Kalko; Echolocation by Insect-Eating Bats: We define four distinct functional groups of bats and find differences in signal structure that correlate with the typical echolocation tasks faced by each group, BioScience, Volume 51, Issue 7, 1 July 2001, Pages 557–569, https://doi.org/10.1641/0006-3568(2001)051[0557:EBIEB]2.0.CO;2

Ulanovsky, N., &amp; Moss, C. F. (2008). What the bat’s voice tells the bat’s brain. Proceedings of the National Academy of Sciences of the United States of America, 105(25), 8491–8498. http://doi.org/10.1073/pnas.0703550105

Roverud, R.C., Nitsche, V. &amp; Neuweiler, G. J Comp Physiol A (1991) 168: 259. Discrimination of wingbeat motion by bats, correlated with echolocation sound pattern https://doi.org/10.1007/BF00218418

Hearing by Bats (edited by Richard R. Fay)