I would argue, and I sense most players are doing it anyway, that you should actually complete the level and get to the status screen to be awarded a talisman. Granted, that's because there isn't an in-game announcement, so they fully complete the level under the given conditions, just to be sure. If there are two talismans for a single level, I think it's more likely that they would be attempted on different takes, especially if they're mutually exclusive (this, of course, depends highly on what the talismans require). Personally, I'd like a loud splash screen since most level packs only have one talisman per level at most, and ultimately not that many talismans compared to levels, hence it's a rare occurrance.
Of course, it's your game, and what's best should be left to you. Either way, I sure hope to see something new for talismans and will be grateful for any acknowledgement for being an achievement hunter!