I doubt there is a perfect method. A method could be to check the date of creation of the filesystem:
sudo tune2fs -l /dev/sda1 | grep 'Filesystem created:'
Filesystem created: Thu Mar 5 15:51:50 2015
The system I pulled this from was created on March 5th 2015. Of course it is entirely possible to install 14.10 on March the 5th and then upgrade to 15.04 so it is no perfect method.
Since ...
cat /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=15.04
DISTRIB_CODENAME=vivid
DISTRIB_DESCRIPTION="Ubuntu 15.04"
the system is 15.04 I would assume it is an installed version and not an upgrade. In hindsight: March 5th probably was the day of the 15.04 got frozen (?)
You can also check ...
/var/log/dist-upgrade/
total 212
-rw-r--r-- 1 root root 1786 apr 24 2015 apt.log
-rw-r----- 1 root adm 169074 apr 24 2015 apt-term.log
-rw-r--r-- 1 root root 33191 apr 24 2015 history.log
-rw-r--r-- 1 root root 0 apr 24 2015 main.log.partial
-rw-r--r-- 1 root root 0 apr 24 2015 term.log
history.log
will have lots of information. That file might be purged on your systems though, but if present will show you if it was an upgrade.
If the logs survived, they could show, for example in a system upgraded from 12.04:
$ grep lsb-release: /var/log/dist-upgrade/main.log
2015-03-14 10:18:36,284 DEBUG lsb-release: 'precise'