Is it incorrect to tack a question mark onto a declarative sentence in order to make it a question? People do this when they speak all the time, but I got made fun of for writing a sentence with a question at the end.
Example: "You got fired for stealing office supplies?"
Is this wrong or only appropriate in spoken english where inflection can play a part?
It certainly works with spoken English. As your question is very likely to be somebody's words, I judge it to be okay in writing too; i.e. the written text of spoken words.
I hope that makes sense. Can you think of an example when a declarative sentence would be used interrogatively when it isn't spoken words? If you can, I suggest a properly formed question would be more appropriate.