Knowledge checks are the way to go for this one. At the beginning of combat, players should be pretty well trained to ask "What's the knowledge check for that thing over there?"
Knowledge checks take no action:
Action: Usually none. In most cases, a Knowledge check doesn't take
an action (but see “Untrained,”
below).
Relevant section on the DC for monsters:
In many cases, you can use this skill
to identify monsters and their special
powers or vulnerabilities. In general,
the DC of such a check equals 10 + the
monster’s CR. For common monsters,
such as goblins, the DC of this check
equals 5 + the monster’s CR. For
particularly rare monsters, such as
the tarrasque, the DC of this check
equals 15 + the monster’s CR or more.
A successful check allows you to
remember a bit of useful information
about that monster. For every 5 points
by which your check result exceeds the
DC, you recall another piece of useful
information.
Typically the first bit of information they'll want is "Type/Subtype". They can ascertain the approximate type without the check (it is what determines the Knowledge check after all), but if they don't hit the DC they don't know what the heck it is. Now, another player could make the check and shout "That's an x! Fangulor, murder it!" to get around the problem.
Additionally, the Bane ability says nothing about the Inquisitor knowing what the creature thing is (and heck if it's disguised, they might get it wrong).
In summary, if no one in the group can hit the DC for the knowledge check, the Inquisitor can't use their Bane ability. Unless of course as @Erik and @Ace point out the Inquisitor guesses the type correctly. :)
Sources: Knowledge Checks, Bane Ability
