If you wipe a small spot with acetone and its color transfers onto the rag or paper towel, it's almost certainly ABS. If the surface softens a bit but doesn't transfer color, it's probably polyethylene. There are other tests for other plastics but your tank is almost certainly one of the two so that's an easy place to start. Obviously don't dump a bottle of solvent on there. Just wipe a small area (near the top/non-structural part) with a rag for 5 seconds or so.
Another clue is how the tank is made. ABS can't be roto-molded easily, while polyethylene is harder to bond with heat. If you see "plastic welds" (melted-looking beads like how metal looks when welded) at the corners it's probably ABS. If it's very smooth especially in the corners with no seams at all, PE. Almost all roto-molded tanks are PE, and almost all oddly-shaped, welded-up tanks are ABS for this reason.
There are some other plastics and even variants of these like HDPE (usually white - your typical cutting board material). These are just simple tests and not guaranteed to be 100% accurate but they'll give you a good start.