You should check whether the original question is tagged correctly. The tag will provide the hint for the syntax highlighting to work. There are some cases where the tag is there, but the syntax highlighting is incorrect - in which case manual language hint is necessary.
Make sure not to approve any edit that is including the language hint where redundant (the code is highlighted correctly in the first place). You can also reject edits where re-tagging is the better choice. Personally, I only mark the edit as helpful if the syntax highlighting cannot be improved by retagging.
Check whether there is anything else that can be improved, other than syntax highlighting. Usually there are things such as salutations, grammar errors, punctuations, capitalization that can be improved.
As a tip, you can always open the original question and look at the syntax highlighting, and hit the improve button to see how the syntax highlighting changes with the edit to compare.