Delphy of Mod the Sims made one of the most valuable and popular tools for The Sims 3 - you can open Sims3Packs (S3P's) with it and (theoretically at least) it will identify if there is any custom content included. It's known as Custard. To see more information about this tool, check out this link at MTS:
As time has passed and EA has changed the way their files are coded, it has sadly become a little less reliable. All the thumbnails will show as corrupt now if you check a S3P after downloading it to your PC, for example, because now all thumbnails show as corrupt. It's still an immensely valuable tool that I would never go without, especially since we have no alternatives! But identifying custom content has definitely become more of a challenge.
Let me just add here that I am not anti-CC in the least, and use some myself, but I prefer to use package versions in my Mods folder because I have a lot more control over it that way. I am very disappointed with how EA has dealt with CC in TS3. Or more accurately, how they've NOT dealt with it. I'm quite sure they are purposely making it difficult to manage because: 1) they want players to buy their custom content from the EA Store instead; and 2) they want to wean players off of CC since they are probably heading toward an all online TS4. But all that is another story for perhaps another day!
Back to Custard and trying to identify CC in S3P's before installing them. The original version of Custard is on page 1 of the above thread; there is another version of it on page 9. I used both versions of Custard to conduct a little test with a lot that I downloaded from the Exchange.
First I downloaded the file using "Save File" so it was saved to a folder on my PC and then checked it with the original version of Custard (which from now on I'll just call Custard 1). The screenshot below shows what Custard 1 identified. All the red corrupt files are png or picture files - it's normal for Custard to show pictures/thumbnails as corrupt now. But it also mentions an "UrnstoneHuman". EA certainly has urnstones in the game, so is this really CC? All of the .png (picture) files are mentioning the urnstone. Also, I believe this lot has some photographs from a photographer sim and if one of them shows an urnstone, it will be identified by custard as if it is CC. I have tried uploading lots with photographs that my tester sim took and they were also identified as if they were CC. This is very frustrating since photographs would be much less expensive (and there would be more variety) than the paintings in the game. I do wish there was a way to avoid having Custard identify photographs as CC. Anyway, the last 3 files are .package files and are CC (or something) that has camped onto this lot.
Here is a shot of the exact same lot using the version of Custard from page 9 of the above thread (I'll call it Custard 2 from now on). Now it is no longer identifying the urnstone as CC. Picture files are still showing as corrupt, of course. And it is still identifying the last 3 files as package files.
So next I added the lot to my TS3/Downloads folder by clicking on "Add to Game" instead of Save File. This opens up the launcher and downloads the S3P. On occasion things will automatically install but most of the time it is not installed to my game until I click on the little box next to it and press the Install button in the launcher. So I can still check the file with Custard before checking that box and installing the lot to my game. The picture below shows what Custard #1 found when I checked the file in my TS3 Downloads folder. It is still identifying the urnstonehuman, but now there is another package mentioning trees, as well as a pattern and a Glove.
As it turns out, the trees thing is a lot that is also available on the Exchange. It has no CC in it, I checked. But somehow it has attached itself to this lot and is installing itself along with the lot we are testing. I've never seen that happen before!
And finally, here is the same S3P checked with Custard 2, which is showing the Christmas tree lot as corrupt along with the 'package in a package' message. Very bizarre, but at least it does still show the custom content that is in the lot. Both the pattern and the glove were created using TSR Workshop. I know this because someone else on the EA forum installed this lot and showed those items that were in her launcher and installed in her game.
So I guess the lesson to be taken from my little experiments is that it is best to use the "Add to Game" option when downloading from the Exchange so that the S3P can be checked with Custard before installing it. I guess whether someone wants to take a chance on something like the urnstone being an in-game photograph would be an individual decision.
If in doubt, you could always go in your My Documents/Electronic Arts folder and rename your The Sims 3 folder to something else, such as The Sims 3 Play. This will make the game generate a brand new The sims 3 folder. If in doubt about a S3P, you could install it with the launcher (it will go to your new folder but anything else you have downloaded/installed, Store content and all, will not be there) and then look to see what all got installed with the lot by switching the launcher to the Installed Content tab.
To delete any unwanted CC from your game is another topic that maybe I'll try to explore/address in another post. I've certainly experienced the frustration of trying to get rid of some custom content, but I have found ways to do it most of the time. You can also always start over with deleting or renaming your TS3 folder as mentioned above, but if you have a lot of Store items or lots and sims that would mean installing them all - AFTER you figure out which ones have the CC you're trying to get rid of (you don't want to install it all over again in your new TS3 folder!).