macOS Catalina, due to ship any moment, is fully 64-bit and won’t run any of your old 32-bit Mac apps. Which ones are 32-bit? There’s a convoluted way to find out via the Apple Menu > About This Mac > System Report > Applications, but there’s also an app for that.
macOS Catalina (10.15) and later versions of macOS will not support 32-bit apps and other software. Before upgrading to Catalina, you can run 32BitAppCheck to get a savable, searchable list of these 32 bit software. To collect the information, just click the Collect Info button, and be patient for a couple of minutes.
Works as advertised. Free. If you prefer a less geeky and nerdy user interface, try Go64; also free.