Virtualbox guest additions allow better integration between the host and guest operating systems like seamless copy/paste, flexible screen resolution of the guest os etc.
If your guest OS is a linux like ubuntu or fedora and you want to check if virtualbox guest additions are installed or not then you can use this simple command
$ lsmod | grep -i vbox
If the vbox modules are loaded they're installed and working. If nothing appears then probably virtualbox guest additions are not installed.
The output could be something like this for example
$ lsmod | grep -i vbox vboxvideo 12612 1 drm 264190 2 vboxvideo vboxsf 43659 0 vboxguest 214867 7 vboxsf