Do you know the specs of the system ahead of time?
1. Verify that the specs advertised match what you're seeing. CPU-Z &/or GPU-Z can be good tools here.
2. Confirm temperatures are in check. Run a stress test and monitor temps. HWMonitor coupled with Aida64 and Heaven benchmark would be pretty comprehensive.
3. Actually test it, doing what you want it for. If gaming, fire up a game you intend to play and see how it performs.
As an FYI, just because it may "work" at the time, doesn;'t guarantee long-term stability.
On components like a CPU etc, that's generally fine.
People tend to cheap out on their PSU though, and over time, that can be problematic.