Let's see...
[] Try to sense his spiritual pressure.
Pesquisa. And if he's that fresh, he won't be able to hide it well... Although... The fact he can go invisible might imply, along with his lack of skill, that before becoming an Arrancar he had such skill. And THAT is why he is still fresh in other methods or masking his body language, because he was good at being hidden in general...
[] Let him stab you, then counter.
Hierro. And again, if that guy is invisible, he might have some stealth kill based abilities, too.
[] Just look really hard.
[] Listen for his movements.
If he has speed, he might be too fast for us to do stuff.
[] Feel the air currents.
CORRECT ME IF I'M WRONG, but I think at the levels of speed that Bleach reaches, this will do jack shit.
[] Throw Getsugas everywhere.
Do we even have ENOUGH power and energy to just keep doing it? If he dodges them, using his invisibility, we might not be strong enough to fight him off even if we know how to fight.
As for the previous votes? If we hold off on escalating, waiting for him to make first move, we might make mistakes. Stalling for time? Remember that sensing stuff outside of the walls is harder, so Nel might not be aware of a fight until it's too late. So we must finish this quickly.
[X] Yes, immediately escalate.
[X] Throw Getsugas everywhere
EDIT: Okay, edited. Because I realized that doing this is also best way to increase our stamina and such, and it is possible to be fast enough to attack a person BEFORE Pesquisa's pulse returns. Zangetsu proved it in fight against Ulquiora