If you have an EC meter, you can tell how concentrated the nutrients are. Now, it won't tell you how the ratios of nutrients change over time, but it'll at least take care of the evaporation problem.

The question as to whether tap water will do depends on your tap water. Get it tested. At home, our tap water has an EC of about 240uS. At my office, it's over 1500uS (deep well water). The sodium is at unsafe levels in my office's water, so I can't use it. Instead, I use rainwater and air conditioner condensate. At home, I could use it, but I'd have to subtract (from what I add) the minerals that are already in it.