Since there's no warning nor error thrown, I assume it's valid AHK code?
This sentence is not correct because no negates everything that follows. You therefore add things to the list using or, not nor:
I went to the supermarket, but they had no (apples or oranges or lemons)
The following sentence is therefore correct:
Since there's no warning or error thrown, I assume it's valid AHK code?
There are differences between British English and American English: it is possible to find occasional instances where nor is used after no in American English, mainly in older publications.
I passed no water nor anything that indicated it. - Southeastern Wisconsin: A History of Old Milwaukee County, John Goadby Gregory, 1932
The third extract contained no gold nor selenium -US Geological Survey, 1914
If you do want to use nor in this sentence, there are two ways to make it more generally acceptable: by replacing no with neither, or by limiting the scope of no using a comma.
The more conventional approach is to replace no with neither. This works because neither just negates the first thing in the list:
It's neither fish, nor fowl, nor good red herring (traditional English expression)
Since there's neither warning nor error thrown, I assume it's valid AHK code?
The more literary approach would be to put a comma before nor, to limit the scope of no:
Since there's no warning, nor error thrown, I assume it's valid AHK code?
Normally, when you make a question, you [invert] the subject and auxiliary verb. A question without inversion is indicated only by intonation. Section 32 of the Oxford Guide to English Grammar (John Eastwood, 1994) has this to say about questions without inversion:
In informal conversation a question can sometimes have the same word order as in a statement. The question has a rising intonation.
The machine gives change? ~ No, it doesn't.
You're travelling tomorrow?~ Yes.
The car is blue?~ That's right.
The car is what colour? ~ Blue.
They went which way?~ That way.
We use this kind of question only when it follows on from what was said before.
I need a return ticket to Paddington. ~ You're travelling when?~ Tomorrow.
Two points to note: it is only used in informal conversation, and only used in a follow-on question. This is more restrictive than in some other languages.
Ellie K's example would only work in a follow-on question:
Person A: A dog bit my leg.
Person B: A dog bit your leg?
Here is a version of the OP's sentence as a follow-on question:
Person A: If there's neither warning nor error thrown, What would you do?
Person B: I assume it's valid AHK code?
In a formal conversation or writing, you should make a question by inverting the subject and auxiliary verb. To ask a polite question, use should:
Since there's neither warning nor error thrown, should I assume that it's valid AHK code?