You can get a tension gauge, it should come with a chart of tension values. Usually, you want it to be around 10% of breaking strength for a particular size of cable.
That said, you set tension and rigging
not to 10% of breaking strength per-se, but to proper mast
geometry AND tension. That means that some will be slightly higher or lower in value as needed to create geometry optimized for your boat
, skills, handling preferences.
It can be done on the water
or on land, but you need to be careful not to have significantly different tension on various sides. You want them to always be within a reasonable small difference of each other.