You could create a parser for each language you want to detect (language definitions for ANTLR are usually easy to find), then run each line of the question through each parser. If any line parses correctly, you probably have code.

The problem with this is that some english (natural language) sentences may parse as code, so you may want to include some of the other ideas as well, or you could limit positive results only if more than one or two consecutive lines parse correctly with the same language parser.

The other potential issue is that this will probably not pick up pseudocode, but that may be OK.