What preposition do we use with the adjective 'telling' when it means 'revealing'?

Telling in this sense is normally not used with a preposition, because it's an adjective.

Public opinion of its culture is telling.

If you have a sentence like your first one where telling can't be used as an adjective like this, it's not a sentence where it would normally be used.

One can use nothing (possibly best)

a telling smile

one can use of (common)

a telling analysis of motivation in business.

This is telling of the high-input crop production and high animal density polluting agricultural practices that continue to dominate the European landscape.

or, one can use about (though this is the least formal)

how telling this is about the culture of the US