How do i know which affixes are global, limited to the item, or player?

For example:
https://pathofexile.gamepedia.com/Sire_of_Shards

is the x% increased projectile damage is for the gems on the staff? for the player only? global up to include minions? Or skills that require the use of a staff weapon to cast? Pressing ALT doesn't tell me more details.

the general rule is that if the mod can be local, it is local. if it cant, its global. for example local mods of weapons are physical/elemental damage, attack speed and crit chance (the mods you can see on white item). there are some exceptions but there are only few of them.

minions are different entities with their own stats and skills. they are never affected by any of your modifiers unless specifically stated otherwise (minion, allies)

The easiest way to tell is by looking on poedb. It's not foolproof but it gives you a better idea than trying to apply that rule of thumb. Increased projectile damage is always global, even if it is on wands which fire projectiles to attack.

'Global' means affects all of the player's skills. Not their minions or allies. The increased projectile damage will increase damage with any projectile skills you use, including spells, attacks, and even your totems, traps or mines since they use your skills.