Foolproof Pectin test
There aren't many good visual signs. What I'd suggest is to add pectin gradually and take samples.


Bring the kettle to boil,
add less pectin then you would expect to add totally (say, half),
take a small amount of the jam, put it in the fridge to  let the pectin settle quickly,
see if it requires more pectin; if so, add a small amount and continue with step 2.


It's pretty much alchemy if you don't have sugar profile of your strawberries or supplier information about the pectin.

You might want to add some thermal insulation around the kettle so you don't burn unnecessary amounts of energy keeping it hot before you get the ideal amount of pectin in.