Phrase Meaning – How to Use ‘I Don’t Know If You Can Tell or Not’ in Spoken English

The ordinary phrase in US English is "You know what?" It is spoken with a falling intonation on know and a strong rising intonation on what:

Ya kno↘w wh↗at?

This is employed to announce that you are about to reveal some exciting piece of information, presumably new to your hearers:

You know what? —I won!

It has the prosodic contour of a question, but it doesn't "expect" an answer: it's a rhetorical question. But the speaker may pause to build suspense until they get an answer, which is normally "What?".

The "what" piece may be expanded to a full free relative clause, as in your Italian example, and in that case "what" may be replaced by another more appropriate interrogative:

You know what happened? —Bob won the 440!
You know who I just saw? —Bob!

In these the pitch-stress moves onto the piece after what or who:

You kno↘w _→ what h↗app^→ened?
You kno↘w _→ who ↗I ^→ just ^→ saw?

There's also a version with the opposite intonation:

Ya kno↗w ^→ wh↘at?
Ya kno↗w ^→ what ^→ happ↘ened?

This is typically employed to announce new information which is not exciting but unexpected and perhaps unwelcome:

You know what —we've been working on the wrong end.
We've completed our study and you know what —Bob was right all along.

The second sentence does sound unnatural. "Tell" needs an object in this case.

The first and third sentences both work, and mean almost the same thing. The third sentence focuses on telling you something. Out of context, it sounds more like a direct warning -- "Hey, pay attention to this, car mechanics are trouble!" The first sentence sounds more like a remark or opinion -- "Speaking of car mechanics, I've met a lot of dishonest ones." It might be agreeing or disagreeing with another sentence. These are only small differences, though.

Also, you might want to put a "the" before "car mechanics". It makes the sentence sound smoother since you're talking about specific car mechanics (the ones you come across).