Learn Spanish (Latin America): Uno nunca sabe con un falsificador inteligente. - One never knows with an intelligent forger.

All three are used for general statements, but uno nunca sabe is the most common fixed expression for 'you never know.' You will also hear nunca se sabe, which is slightly less personal. While you can use the generic tú in Spanish (tú nunca sabes), native speakers overwhelmingly prefer uno for this specific idiom.

Why use con (with) instead of de (about) when we are talking about knowing something?

We aren't talking about knowing facts about the forger (saber de). Just like the English 'you never know with...', the preposition con implies predicting what will happen when dealing with a certain person or tricky situation.

Could I add a phrase like que yo sepa (that I know of) to the end of this sentence?

It wouldn't make much logical sense here. Phrases like que yo sepa or que me conste are used to limit a factual claim. Because uno nunca sabe already declares complete uncertainty, there are no facts being claimed for you to limit! You would instead use those phrases with a definitive statement like: No es un falsificador, que yo sepa (He isn't a forger, that I know of).

How is the noun falsificador constructed?