It's often tough, because you want to separate the effects of the blowout (long rip in the inner tube) from what caused it. As the tire demounts, you can get a long tear in the tube that is far away from the initial failure site, or a small hole may rip the sidewall as it explosively decompresses.

Is the outer case cut cleanly at the gash? That would support a bit of glass or metal cutting the tire. Not much you can do about that.

Is there scuffing either side of the gash? Is the wheel true, or does it wobble as it spins? If there's a wobble, you may have been rubbing against a brake (especially a brake shoe, if the brake pad is worn down), or a fork or stay, or whatever else you may have hanging on the bike.

Take a careful look at the bike; is one of the brake shoes shiny and all the others are dirty? That may indicate rubbing the brake shoe rubbed the sidewall down until it ruptured. Is the paint worn away on a fork, chainstay, or seatstay near where the tire spins? Maybe that's where the problem is.

Look carefully at the tread of the ruptured tire. Is there a slice in the tread? Or is there threads or rubber of a different color showing? Has part of the rubber peeled off? Also check the tire bead; is the bead solidly attached to the sidewall all the way around the tire? If not, it's possible the tire was damaged removing or installing it, for instance when you fixed a flat.

If you don't see any warning signs, you might refer the evidence (tire, tube, and bike) to someone experienced to help track things down. It's possible to have roadway debris slice a tire, but poor tire mounting and worn/damaged tires are more likely to cause a blowout IME.