Well, it does take persistence. My brain is pretty scrambled on many things atm. Of course I do miss my physical books. When I do research I do use a computer and have like 40 windows open but I also have like 20+ actual books open too.
Then its like a random butterfly and a kitten and I get distracted or get slump and take a nap or w/e.
What we know about illusion:
They're not real in most senses or rather they confound the senses with misdirection. I mean we don't really need to debate what is real or not real as that's like saying robots have souls and holograms deserve human rights (which screams star trek and other odd shows/movies).
Illusions vary in degree and power affecting these senses right:
Wiki Senses
Sight - Vision
Hearing - Audition
Smell - Olfaction
Taste - Gustation
Touch - Tactition
Then there are parts of the senses as per that simple article. There's also that gut feeling sense cops get or the extra sensory powers I guess.
Bard's Tale had it simple. Just make an illusion mob that can be disbelieved via int/wis/level check. Or a spell. There were monsters that were disbelieved though spells weren't.