Here's the wiki page on finding problem custom content:
http://www.simswiki.info/wiki.php?title=Game_Help:Finding_Problem_Custom_ContentPersonally, although it is very time-consuming, I lean towards the tried-and-true 50% method, as mentioned above and also at the link.
Just remember, if at all possible, to
not Save your game while testing with content temporarily removed -- just Quit without Saving.
Oragnizing your Downloads folder into sub-folders can help speed up the search somewhat, as can segregating the newest items you've put into Downloads (you can always move them to a diffferent appropriate sub-folder after determining that none of them are causing problems).
As far as checking for hack/mod conflicts, there is the HCDU (Hack Conflict Detection Utility), here (click Sims 2-- Programs):
http://www.simwardrobe.comNote that the HCDU (a) doesn't necessarily recognize 100% of hacks as hacks and (b) will show a list of potential conflicts, but cannot determine which if any of those potential conflicts are benign and which are malignant (no program can do that).
Some tips:
If you've recently installed a new EP or SP, some items which previously caused no problems may no longer be compatible, and need to be either updated or scrapped.
If the game freezes or crashes while loading, the problem may not necessarily be custom content. There are other tricks to do to (usually) solve that type of freesing/crashing.
Collection files (and any image files which accompany them) in the Downloads folder can cause crashing -- those files belong in the Collections folder, not the Downloads folder. Running Clean Installer and sorting the files by Type can quickly find any Collection files which may have ended up in Downloads. Clean Installer can be gotten here:
http://sims2pack.modthesims2.com/Highly recommended to always use Clean Installer for installing Sims2Pack files rather than the installer which comes with the game. Also recommended to update to the most recent Clean Installer in case you should happen to have an older version already installed.
Generally speaking, the only types of files which belong in the Downloads folder are .package files. Any .jpg, .png, .bmp, .gif, .zip, .rar, .doc, .html or .txt files don't belong there, are ignored by the game, and are just uselessly bloating the folder and needlessly taking up space. You can either move those other types of files to someplace else on your computer if you want to store them, or else delete them.