The Cassini radar is quite capable of detecting a flat liquid surface (given light winds or less, and a proper angle).

Also the two instruments that are able to image through the clouds are capable of detecting specular reflections (lining up with sunlight - proper angles again).

Given that the clouds in the south seem to be something more complicated than methane, then predictions of what liquid bodies on the surface might be made of is now more complicated.

Also, temperature ranges on Titan certainly include the triple points (or double points) for all sorts of hydrocarbon gasses and solvents. And there are a lot of hydrocarbons in that part of the universe! Nitrogen, too. The only question is whether conditions exist where such liquids can be replenished as rapidly as they are changed into something else.

They have only examined less than 1% of the surface for liquid, yet. Give them a minute, guy!