All compost piles leach. This is ok. It's not a problem by itself. The question is what happens when you overload what the soil below can handle. Keep in mind that plants and even bacteria on the surface of the soil, cycle and transform nutrients. You can "detect" this by simple observation of nearby plants. Moderate (ok) levels of leachate may cause plants next to the pile to grow better than those around them. If you see a dead zone around the heap, you may want to back off....
I don't know that there is a single definition of where problems might lie, much less a simple way to detect it. However here are some things you can do to minimize causes for concern.
If you are processing non-manure, there is still some risk of fecal choliforms but this is minimal. There are nitrates, but plants will suck these up if they are not excessive. Most of these are not threats to groundwater at any depth that one would get from a well. If you are composting a lot you might.....
Some things that permaculturists sometimes do that would reduce the risk further include:
In-place composting. The compost heap is in the garden, a new one gets started in a different place every year,a nd the old material spread around where it was composted. If you overdo it, I suppose your plants won't grow and that will be a warning sign.....
Moving the existing compost pile once a year or so and growing plants where the old one was. This helps use the leachate in the soil and it provides some controls to prevent overdoing it.
Edit: My gut feeling is you'd really have to be doing industrial-scale composting for this to be an issue.