This is what happens when you refuse to accept takeback

which was clearly a mouse slip, wanted Qg5 my opponent resigned and left the game so fast probably due to embarassment

or because he had no reason to play a game like that

You are implying that people deserve to lose or should be punished for refusing a take back??

People deserve to lose if they ruin a potentially good game by trying to capitalize on mouse slips. Takebacks are meant as an option to correct such accidents. @CeremonialOath was an agent of chess karma in this case.

how can you tell if it is a mouse slip or it is a blunder i never ask for takebacks or accept them

Mouse errors are blindingly obvious...treat them as you would an accidental piece-shift in O.T.B. such as your opponent's shirtsleeve brushes a piece he obviously was never intending to move and he adjusts it without a declaration of intent.
Commonsense,really.
And that almost extinct thing...Chess Etiquette.

Can’t they move forum postings containing the word „takeback“ in the kindergarten section?

ure righht, sargon

To be honest, 19...Qh4 does not look like a doubt-free mouseslip.

Meanwhile, your opponent completely lost the thread after he may have been distracted by your takeback proposal.

Really not a game I would brag about.

Huh? Being able to use a mouse is part of online chess, especially when it comes to blitz or bullet. Just like with any other relevant skill, if you're not perfect at it you'll lose some games to it. Though lichess went as far as implementing a confirm move option so even if you are lacking in that you can still fix it.
@bunyip "such as your opponent's shirtsleeve brushes a piece he obviously was never intending to move and he adjusts it without a declaration of intent." What? That's not how it works, when you adjust a piece you always say J'adoube before, at least in a semi serious setting.