32 Bit plugins usually end in .8bf. There are some that are .exe like Eyecandy, and Alien Skin, when you install them you will see an old and often blue computer screen. These should be installed in the Program Files <x86> folder (32bit) Majority of the old plugins are 32 bit. There are more older plugins around than the new ones.
This site links to many of the original sites, where they may provide information on the plugins and also will link to the some of the linked sites may have the new 64Bit versions (This site also includes some freeware plugins for download (mostly 32bit). http://grafnet.com.pl/
Some of the old plugins can be “patched” using a program called Filter Meister, (FM PAtcher) so that they would work on new PCs, however, it can be a tedious process when you have many filters to do, and some of them do not like to be “patched”. I have seen some of these filters have the words “patched” & 64Bit in their filenames.
This site has some useful information on plugins and the installation of plugins inc DLLs.
There is another program called Filters Unlimited (versions 1 & 2) where some of the old filters are imported into the program database, and will work on newer PCs.
Flaming Pear, Richard Rosenman & Alien Skin that now has 64 bit Versions of their plugins.
Alien Skin is now Exposure Software