This is darn difficult. My attempts at similar projects have not done well. I tried to use a dye that is very strong, a little going enuff to dye a lake and with slow leaks, not good. I tried to release small amounts of dye from a tube next to the bottom and from a boat and a viewer (blocks out side light) watched to see if it was leaving into the liners. No luck.
I suppose one might try the reverse. Empty the tank. Install some well points around the tank. Dose the area with dyed water. Look inside to see if it shows up. I'd not be too confident that it works.