Guest additions technically consist of device drivers and system applications.
Telling if guest additions are installed
Hostkey + n opens session information whose Runtime Information tab lists the Runtime Attribute Guest Addtions along with its version number. The version number is reported as Not Detected if they're not installed.
In a Windows guest, the following
PowerShell command might be used:
get-service *vbox*
In a Unix guest, the following command allows to verify if guest additions were installed:
lsmod | grep -i vbox
Also in a Unix guest, the guest additions are (always?) installed under
/opt/VBoxGuestAdditions-x.y.z.
On the host, the command
VBoxManage showvminfo $vmname prints some guest addtions related information under the sections
Guest and
Guest Facilities.