Alternatively, a simple Creo Vim (base 1), Ind, Sun (+2), Sight (+3), 10, let's call it "Touch of Hermes". You simply put a magical effect on the target, with no other purpose but to taint it with a little bit of magic, yet weaker than the Phantom Gift so there is no side effect. Then an appropriate InVi will allow to detect it.
There is no guidelines for detecting personal spell similar to the one for your own illusion, but I believe it is reasonable to design such a spell, which would be of lower magnitude than a generic magic detection spells.
So I would go with "Discern Own Magics", InVi 15 (a copy-paste of Discern Own Illusion, with a Base 1 effect: detect his own magic).
Otherwise, it would be a Base 5 Intellego Vim spell, +4 for Vision, +2 Sun, so a level 35 (I removed the +2 for Techniques & Form, and bump the duration to Sun).
Second option, as you proposed:
You cast a minor illusion spell on the target (or on his clothes), and you redesign Discern Own Illusion, but instead of making your own illusions more transparent, it amplifies it - it is a cosmetic change since it does not change fundamentaly the functionality of the spell.
"The Faint Glow", CrIm base 1, duration Sun (+2), Sight (+3), for a final CrIm 10 (add +1, if you want to be able to target a Part - like a hat on a person), allows you to give a tiny glow or shade to the target. It is only perceivable in darkness or in strong lighting, depending on the selected effect.