Sometimes - on a close coupled toilet where the cistern sits directly on the WC pan - a leak can come from the seal between the two pieces - noticeable when you flush.
The seal is made with a large squidgy washer called a Doughnut Washer. It's held down and compressed by a metal plate. Two bolts with wing nuts apply the tension to make the seal.
You can often fix a leak here by tightening the wing nuts on the bolts. They hang down at the very back - one each side.
Otherwise call a specialist in...
Good luck...
Peccavi, May 2011
