Although this won't help for past water leaks, in the future have Maintenance show you where the water shut off is for the toilet/sinks in your unit (toilets will flush by pouring a bucket of water into the bowl unless it's plugged). If you suspect a water leak, you can put any colored cleaner in the bowl (for instance Pine Sol or Fabuloso) if there is a leak the color will gradually get lighter.

Always leave a paper trail for leaks. If your leak starts after hours, call your complex's after hours emergency line - they can decide it's not an emergency and you have a paper trail.

The advice to contact the water company with your maint. order is a good one. I hope you do that. If the water company won't work with you, they will at least set up a payment schedule for such a large amount. You'll have to find out the tenant's rights in your state to see if you can claw that back from your apartment complex.