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I'm curious because I remember seeing something on this but I can't find it now.
I'm wondering because I've got a group of these elite assassin/scout/spies, who use a little magic and psionic power to make themselves really dangerous. But, say one throws Telepathy to scan a party member, would they (the PC) know? If they fail the save I'd assume not, but we've got a minor psionic and a Mind Mage so they might catch on if somethings up.
The main psionic powers in question are See Aura and Telepathy. Though any information would be greatly appreciated.

If the individual has Mind Block Auto-Defense it will kick in when they are being probed blocking those powers in question, and give them indication they are being probed. However outside of that I don't think anything gives away when either power is in effect (at least when Telepathy is in "receive" and not "transmit"). If a person suspects they are being probed, they can resist to get the standard saving throw. Otherwise if they have access to the regular Mind Block (and/or Group Mind Block) they can erect it as a preventative measure.

Group Mind Block mentions that you can't know its in place w/o a "detect psionics" (pg174 PF2E), though I suspect other powers would also give it away. Unfortunately the ability isn't part of any PF2E Psi Powers in the main book.