If you look in the core rulebook, in the Magic chapter, in the list of spells at the start of the Illusion section (pg 290), you'll find out. I'll post it here for you.
Mana Illusions: Mana-based Illusion spells affect the mind and are ineffective against technological viewing systems like cameras. Mana illusions are resisted by Logic + Willpower. Some mana illusions affect the target’s senses directly; others affect the senses of anyone perceiving the subject of the spell (though you’re never affected by your own illusion spell). Though mana-based illusions can be created on the astral plane, their magical auras give them away as illusions to anyone who makes a successful Assensing Test—illusions can’t fool Assensing and cannot be used to disguise or create auras.
Physical Illusions: Physical illusion spells create actual images or alter physical properties, such as light or sound. Physical illusions can be used on both living and technological systems. They are resisted by Intuition + Logic; non-living devices resist with their Object Resistance. The magician must generate more hits than the observer for the illusion to be considered real. If the spell is not completely resisted, the target is fully affected by the illusion.
Then you look at the spell being cast; usually for Illusion spells there's a Mana version (Mask, Invisibility, Phantasm) and a Physical version (Physical Mask, Improved Invisibility, Trid Phantasm). The threshold the resistance rolls have to reach is the hits scored by the spellcaster on their initial Spellcasting + Magic [Force] roll.
Keep in mind, Illusion spells cast right in front of people can be self-defeating-- As per the rules on page 280-281, whenever a spell is cast, it produces some kind of noticeable effect. People can roll Perception + Intuition (with a threshold of (Force) - Spellcasting of the caster) to detect it being cast. This is more than just visual, it includes auditory and maybe even other senses. So a F6 Invisibility spell cast by someone with 6 Spellcasting has a threshold of
1 to notice it being cast; there's some flashy effects and probably a sound or two. This doesn't let people know where the invisible mage is; it doesn't even let them know that
invisibility was the outcome. But it does let people know you were there at least and that some mojo went down.