Fill it with water and try to pressurize it through the cap. You could build a simple test fitting using a rubber "jim cap" (found in the plumbing section at any hardware store) drill a hole in it, screw in a bulkhead fitting with a schraeder valve in it and pump it with a bicycle pump (if you don't have an air compressor.) You'll find your leak pretty quick that way. Should cost less than $10 and could be used as part of the setup for blowing out dents in 2t pipes in the future.
I'm pretty sure the tank is made out of hdpe so you should be able to hot air weld it, or use plastic specific 2 part epoxy available at any home/ autoparts store.