How can you tell if the other side is bluffing in negotiations?

By reading and understanding the behavior and flaws in their communication.

thanks for the invite

you can use the body languages techniques

To be talented in Negotiations you need to study the behavior of the person you are negotiating with.  You need to understand and study their body language which reveals a lot of the hidden things that cannot be revealed by words.

Body language is a great science that people study and add to their skills to achieve this and make a win-win situation.

Thanks 

May be when S-he tells you some lies, and you knew well what the truth is 

I think it goes with experience to actually know whether your competition is bluffing or not. 

thanks for the invitation!

simple.....ask them about their family first, analyze the position then.

then ask them about the work, and the challenges, then analyze

then ask them about the expectation, if its high then ask them the reason...that's it, you will come to know automatically...but be smart while asking.....

You can judge with the body language & face reading of opponenet & facts & figures

By knowing the leverage of the negotiation.

By experience & gut feeling

Thanks for invitation,

It depends on how the negotiation is going on,

- If it is going in a "win win" or "win lose" situation, you can till him after you close the deal in indirect wording.

- If it is going in a "lose win " or "lose lose" situation, you can stop negotation immediatly, tilling him that this due to his bluffing.