12footer,
We can't say with certainty what type of liner you have.
However, IMO it would be rare indeed to use a plastic pond liner in an area that contains heavy clay. I think you should probably work on the assumption of a clay liner.
I have been digging test holes on our property to put in a few small ponds. We have loam to sandy loam soils that readily transmit water. However, we have a layer of solid clay below the topsoil.
I can always tell when I get close to the clay layer, because the soil just above the clay layer is damp, and I always have free water sitting on top of the clay.
I would follow esshup's advice above about determining the level of your clay liner. I suspect you will see an obvious color change and the clay liner will be much more compact on your bucket scrapes.
