From your activity.
If you submit you live in Korea, but your IP address show you're in US, France etc, it will raise a suspicion. If you chat using Spanish language with perfect grammar, it also raise another suspicion.
If you notice a particular user who, each time they login to their account, appears in a new location with a different IP address from their last login, is that not an obvious sign of VPN usage?
This is not entirely correct.
Most people are using dynamic IP address which make your IP address keep changing even you're didn't move to anywhere. Let's say that you're live in Los Angeles, but your IP address could show you're in Washington, Dallas, Boston etc.