Checking spoke tension without specialized tools
On a standard front wheel all spokes (both left and right) should (in theory) make the same musical note when plucked. A rear wheel is 'dished' to make room for the cassette. The non-drive side spokes will be at a lower tension (and pitch) than the cassette side.
If you can find a bike with the same spokes and lacing pattern use that as a guide. If not, any newish bike can be used.
Too much tension will damage the rim at the eyelets, whereas broken spokes are a sign of under-tensioning.