CPU steal time and swap usage tell you what's coming days before an outage does.
Watch swap usage
If your server is regularly writing to swap instead of RAM, it's already starved for memory — response times will degrade well before anything actually crashes.
Watch CPU steal time
On shared virtual hosts, steal time shows how much CPU your neighbors are taking. Rising steal time with flat load usually means it's time for a resize or a dedicated instance.
Watch disk I/O wait
High I/O wait with normal CPU usage usually points to a storage bottleneck, not a compute one — worth checking before assuming you need more cores.