How to see what expansion/stuff packs were used in a save file

@MileenaRenz  There's no way to find this out, unfortunately.  But often a save is grayed out because the world it takes place in isn't installed, and you can see that by opening the save's folder, in Documents > Electronic Arts > The Sims 3 > Saves.  Each world that save has simulated will have its own .nhd file, which is at least somewhere to start.

Another common reason for a save to be grayed out is that the main household contains a pet without Pets installed, or perhaps a supernatural sim without the pack that introduces that supernatural state.  Missing stuff packs will never break a save like this.

Other than the above, the only real way to find out is to load the save with or without certain packs and see whether you get the missing objects warning, and then add or subtract from there.

If you're often going to enable or disable packs, it might be best to have a list somewhere with the packs involved in each save.  I keep a couple text documents in the Electronic Arts folder with info about my current saves.