After I read some of the recomendations here, I did
grep -ri bionic on the root folder of the USB.
I've found a folder .disk on my 18.04 live Kubuntu USB. There is a file named "info" inside which has the following text:
Kubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
This is great because I wanted to know what is the specific "subversion" of the Ubuntu installation (I knew that it was 18.04 but didn't knew if I needed to upgrade it to 18.04.4).
I do not remember how I made this USB and do not know if this folder is present dependable on the method of creating the USB. So try it yourslef and see.