Missing Textures Warning Pop-Up

[frustratedgamers]

I saw this on GMOD Tower and was wondering how this could be done for other servers? Possible with ULX?

If this is possible I'd love to know how to get this done!

Thank you in advance!

NOTE: If you are confused or don't know what I'm talking about, basically its a message that pops up saying "Looks like you are missing textures from CS:S or HL2 EP2" etc... and might say where to go to download the textures and what not.

[JamminR]

The first thing that came to my mind was file.Exists.
Check if some exclusive to <some game> file or folder existed, and if not, give the message.
However, looking over the Gmod wiki shows lots of different libraries that make me wonder if one of them could be used in a better way.