Re: Can a player attack a den without having a recon token on it?
Yes, they can. Doing so will reveal the boss, but in order to score points for doing so they must have a Vengeance card of the matching boss or gang in play. If they have a matching boss card, they can score for both killing the boss (VPs are on the Vengeance card, can score multiples of these at once) AND clearing the den of minions (each den has a VP value). If they don't have a matching boss card, they may only score for killing all the minions in the den and must discard a card matching the gang in the den. That said, they can still kill the boss in the latter case anyway to remove them and their den from play, so this might be a strategic decision on their part.
However, the one exception is Big Bosses - if you enter a den containing one without having a matching Vengeance card, you're booted out and have to sit out that Fight turn. Big Boss cards are shuffled into lower half of the Boss deck, so you can generally have an idea when they're going to start coming into play.