From Seinfeld

Is it “I tell you, …” or “I’ll tell you,…?”

Sometimes I just can’t hear it clearly when they speak very fast.

It could be either as both informal phrases are in use (and often used interchangeably).

‘I’m telling you’ or ‘I tell you’ or ‘I tell ya’ can be used for giving emphasis that what the person is saying is true. Someone could say, ‘this is great cake, I tell ya’ or ‘it happened, I’m telling you’.

Many thanks.