How do you test water?

How can you test excess water to find its source?

• Basically my basement keeps flooding, and coincidentally next door have a continuous flow of water out of their overflow and into the drain. I've called Severn Trent about it, they came around, tested the water and confirmed it was treated water. They were supposed to come again to check out the drain, but they didn't, so I called them again and eventually they sent another guy around. He was here for about 2mins, in which time he asked me if I knew what kind of test the other guy used (which ofc I had no idea), put it down to "natural spring water" and said I just have to keep pumping it as and when required, then left. Now, I'm not doubting this guy's psychic water-origin detecting abilities, but I'm hoping someone on here knows of anything I can do to test the water (or any other service who can) to see if it is indeed treated water leaking into the basement or just ground water...

• Answer:

  Well, if you know the source of the treated water, there are certain UV dyes you can get. Add a couple tablets to the water and then use a black light to monitor the water in your basement. If no dye apperas, you will know it is not from that source.