Quotation Marks and Symbols

This entirely depends on the format. EL&U, for instance, uses Markdown formatting which allows you place specific characters within code blocks as such. This would replace the quotes that are being used to wrap the actual quotation marks:

• Example one uses the characters '"

In common text, however, you are stuck with the available wrapping characters of '"(){}[]. Nesting or escaping this can easily confuse readers but there are tricks that can help:

Please enter "Test" into the field (no quotes)

This is the most common because it is unambiguous with regards to what is being entered but also allows for special characters or multiple words:

Please enter "Test five" into the field (no quotes)

Please enter "90#8mrk" into the field (no quotes)

Obviously, as you mentioned, you wouldn't be able to use quotes with this mechanism. I personally prefer to use new lines for such content (with an indent if at all possible):

Please enter the following text (including symbols):

"f94kk;'4\

Please enter the following text (including punctuation):

"I am John," he said.

This stops working if you need to include whitespace as well but at this point you may as well be talking to a programmer in code.