The main thing to check for when approving projects is to see if the project is "legitimate". Figuring that out, though, is rife with issues.
Is it a UI package of other people's work?
CurseForge no longer accepts UI packs as projects.
Is the submitter the author?
Check to see that whoever submitted the project is actually the creator. This means no fan updates for the general case. Only in extremely rare cases would this be allowed, e.g. if the previous project is abandoned and the license allows for someone else to take it over.
Is it an actual project?
Since there are contests that involve requiring a project, there is incentive for people to create fake projects, so watch out for these.
Check the description, if it says nothing of substance about what it is, don't consider it a real project.
Does this violate any in-game rules?
If you believe that the project violates any in-game rules, find out for sure and make a judgement call. We don't want to condone any project that breaks in-game rules.
Does this violate any non-disclosure agreements?
For projects made for games that are under non-disclosure agreements, they should be deleted. We don't want to make anyone angry, and we will not condone breaking such contracts.
If you suspect that the file uses code obtained through copyright infringement, dig deeper before approving, find out the real story and make a judgement call, perhaps ask another admin their thoughts on the issue. In the general case, err on the side of caution.
Some files may contain viruses or other malware. One of the best checks is to open the file, see if there are any executables or otherwise suspicious files. If you notice anything out of the ordinary about this, dig deeper.
For files that do have executables, it's possible that it is legitimate. In these cases, unless you explicitly know the author, you should talk to them and figure out the real story. You may request source code if you so deem it appropriate. It is also recommended that you run through any such files through a malware scanner.