By my logic the Jedi wouldn't be able to sense that Palpatine was a Sith even while sitting in a room with him where there was no dangers or distractions. I'm no prequel fan, but I recall that it's mentioned the Jedi's ability with the force was being weakened.

And it wouldn't affect the ability to swing a lightsaber around. Though it would affect the ability to use precognition to predict where the opponent was going to swing their lightsaber. I mean they can deflect blaster shots all day but can't deflect a lightsaber that's moving much slower? Why is this? Makes sense if you consider the guys shooting the blasters don't have much force abilities so they can use the force to predict where the shot will be and deflect it. But another force user's actions can't as easily be predicted so it comes down to good old fashioned martial arts abilities.