It's possible that a compost pile will leach in to the ground, possibly contaminating groundwater and nearby wells. If you're composting human or animal manure, the risk is greater, as is the concern of regulatory agencies.
Is there a way to detect whether a compost pile is leaching?
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.
See the other answer for the question of whether this is a real issue.
If you want to mimic an industrial approach, you can put the heap on a plastic foil that's inclined into a small barrel, bucket or whatever to contain the leachate. I'd use PE foil since it's more resistant to chemicals than PVC. Keep away from sun.
Another industrial approach is to put your compost heap onto a concrete or asphalt slab that's drained into a gutter.
Either way, I'd advise to have a grate or something between the pile and the barrier (Foil or asphalt). This way you ensure that the bottom of your pile is aerated.
If you put a lid on the gutter and minimise the area that's open to rain, this will also tell you how much leachate you get (if this is really important information to have, I don't know)
If you spread the leachate over a wider area, you should minimise nutrient entry into the groundwater.
