How can I tell which USB stick I used to install Fedora?

Yes. Start Fedora normally, then plug in one stick at a time and check it in the file manager. The installer may have a Fedora label or contain Fedora installation files, though the exact label can vary.

Plugging it in while Fedora is running won't start the installer. Installing Fedora also doesn't automatically wipe the USB stick, so it should still be there unless it was changed afterward.

You can reuse the installer stick as ordinary storage. Back up anything you want from it first, then format it using Fedora's disk utility. Formatting erases the selected drive, so double-check its size and identity before proceeding.

Some installer images give the stick an unusual partition layout. If a simple format doesn't give you the usable space you expect, you may need to recreate the partition table and then make a new partition.

There's also a good reason to keep one as an installer. You can boot from it later to reinstall Fedora or troubleshoot a system that won't start. You can refresh the image when you want a newer installer, but it doesn't need to be updated constantly just to keep it around for repairs.

Even if the computer does boot from the USB, that normally takes you to a live environment or installer rather than automatically changing your installed system. Just don't proceed through installation or disk formatting unless that's what you intend to do.

If you regularly need several installers, Ventoy is another option. It lets you copy ISO images onto a prepared drive and choose one at boot. It can have Secure Boot or compatibility issues with some images, though, so test anything you plan to rely on.