How to know/decide if it is wrong English or slang or colloquial?

[BobK]

I prefer 'It stinks'; a stink is already bad, so I don't like 'It stinks bad'. But, given that a stink is already bad, it strikes me as OK to specify the degree of badness - so, in the right context, I'd find 'It stinks - real bad' quite acceotable.

b

[Tdol]

Certainly better than extremely bad IMO.

[probus]

I don't think there is anything Indian about saying something stinks bad.

Agreed. But when it comes to stinking bad, India is a perennial superbowl contender. Nevertheless I love the place

[Tdol]

There are some Olympian streets in my area (Metro Manila).

[Olympian]

@BobK, thank you. I did not know that.

[Olympian]

Agreed. But when it comes to stinking bad, India is a perennial superbowl contender. Nevertheless I love the place

You are "bad!". (I mean it in the American sense, of course!)

[probus]

Best praise I've ever received on this site Thank you Olympian!

[Raymott]

Also note that "How to know if a particular expression/sentence is just wrong, or if it is acceptable?" is not a valid sentence.
There are no proper English sentences that go "How to read?", "When to eat lunch?", "Where to go to school?" etc.