The DEFINITIVE answer for simple sloop rigged boats (cruising, not racing boats) is cap shrouds and lowers plus headstay/backstay at 15% tension.
Rig tension not only 'holds the mast up' but also provides a known/predictable 'curve' to the forward part (luff) headsail/jib ... and a sailmaker EXPECTS (for a plain-vanilla cruising-cut sail) that all the rigging is tensioned at ~15% so that the headsail will be HELD IN PROPER SHAPE (by the proper tension in the forestay). If you dont have proper forestay tension you will NOT have any good shape in the jib/genoa. Too loose and the boat will heel aggressively, wont be able to 'point', will be very SLOW, and the keel will be skidding off to leeward (and the wake coming out the stern will be at an angle and not coming off 'straight').
A too loose rig, besides being quite vulnerable to shock and impact loads, will not provide the proper tension that the sailmaker EXPECTED to be in the forestay.
A too tight rig, will be vulnerable to rapid FATIGUE FAILURE --- you really dont want your 'normal' sailing rig tension to exceed 30% tension when the rig IS 'FULLY STRAINED'. Too tight rig tension will also result in improper jib/genoa shape (max. draft too far forward and leech sections 'over-flattened' or falling off to leeward)
Simply looking to see if the leeward shrouds are loose/slack/etc. when on a heel is ........ 'nothing' sensible.
You need either a proper rig tension gage ..... OR use the formulas in the following to set up your rig with **EYEBALLS and a STEEL TAPE MEASURE** .... Rig tuning instructions - adjust your rig
These 'steel tape' / tension methodologies are applicable/appropriate to ALL sizes of rigging wire diameter.