Welcome to the Bunnings Workshop community @newbie25. It's fantastic to have you join us and many thanks for your question.
Had it rained within the last day or two prior to inspecting the property? If so there is a good chance that it is rainwater as that looks like a place it could reside for a day or so after rain. I'm not sure if there is any method you could employ to determine whether it is rainwater or tank water. You could try inspecting again when it hasn't been raining for several days.
You could always ask the owner if it is leaking from the tank. They might not give you an honest answer but I know many people would. My mother recently sold her property and they asked if the rainwater tank was leaking. It was and she offered to have it fixed prior to them purchasing the property.