How to check if you outgrew your VPS plan?

A new parameter has been added to the vmstat and sar utilities a couple of years ago, the CPU steal time. As explained by Rik van Riel, it is

1. the time that (1) the CPU had something runnable, but
2. the hypervisor chose to run something else instead.

On an unloaded VPS, the %steal value in sar or the st value in top should stay below 5%. If it regularly gets higher than 10%, time is to upgrade or to move to a dedicated server.