- All Sorc prestiges present visual indication.
- PM Wizards in form present a visual indication. It's been a while - I think AM is indicated by inspection, but I may be wrong.
- Tempest prestige is indicated by inspection when dual wielding. AA does not present an indication.
- FvS prestige is indicated by inspection unless someone else has been Named Champion. The animation for this act is unambiguous and easily noticed.
- Cleric prestige is indicated by burst or positive energy aura. Possible to have prestige and not use functions, but unlikely, and nearly the same as not having the prestige.
- DoS/SD is indicated on character sheet when stance is active. Possible to have prestige and not use stance, but unlikely, and nearly the same as not having the prestige.
- Kensei does not present indication unless Power Surge is active. Failing to use Power Surge for Kensei 2 (and thereby present an indication) is nearly the same as not having the prestige.
- Frenzied Beserker does not present indication until Frenzy or Death Frenzy is active. Failure to Frenzy and DeathFrenzy is nearly the same as not having the prestige.
- Assassin does not present indication.
- Bard prestiges do not present indication.
The list of prestiges that do not present a visual or inspectable indicator is smaller than the list that does.
It is true that Absence of Proof is not Proof of Absence, however the prestiges that are plausibly relevant to ''albusiswaybetterthanme' (FvS likely, Cleric possible) are more likely to be indicated than not.
For 'derpdualtwizzler' (twf DPS - ambiguous - Assassin, Kensei, Tempest are reasonable guesses, but not certain), best to ask.
Seriously, though. You don't need to be Columbo to derive whether or not a player has taken and is using class prestige abilities.
Whether or not it's appropriate or helpful to badger someone for taking / not taking or using / not using a prestige enhancement is another issue entirely.
Prestige enhancements increase effectiveness in most cases. Seems silly to me to leave low hanging fruit unpicked.