Would Detect Thoughts have a visible display?Exactly. Magic has so little counterplay in D&D as is, I don't want psionics to remove those few guardrails that are left. I'll take the temple-pressing, grimaced face and semi-visible energy waves over invisible and undetectable abilities.
Its an honest question, I have wrestled with this issue for years, the balance is difficult.
We used to have psionics working in anti-magic zones, cause "its not magic", and you couldn't dispel it. Nowadays, we decided that both "magic" and "psionics" are kinda based on quantum physics, so can affect each other. (Cant counterspell psionics though)
And we once had an evil psionic NPC mind blast another NPC to death while casually drinking at a bar.
Now to make a short story/question even longer...My campaign persecutes psionics for that very reason, hunts them like the spanish inquisition. So its mostly NPCs.
But for PCs and NPC interactions, yeah "undetectable" casting is a concern. So we tried on a case by case basis, Detect Thoughts (undetectable), Mind Blast (air distortion), Pyrokinectics (well I'm sure you can see the fire, but can you tell who started it?)
Although not many (in 5e?) DC rolls to check for detecting spellcasting, we gave non-obvious psionic abilities advantage on "subtle spell". Kinda works...kinda cumbersome...
"WE" want psionics to have an advantage in casting, but not totally overwhelming. And we need that flavor to be part of the reason psionics is persecuted (mind flayers, aboleths, etc, are another reason...).

