As we've discussed in several other questions, one of the important bodies from which we derive fiqh (legislative rulings) is ijmaa, or consensus of the scholars.

I wonder, then, how is it that ijmaa is established? How do you know if there's ijmaa on a topic or not? Particularly because some issues have a claimed difference of opinion about ijmaa -- some scholars say "there is consensus on this topic" while others say "no there isn't."

Edit: Here's an example. In the science of hadith, the end result is: "this hadith is da'eef (weak), hassan (good), or saheeh (strong)." This depends on the qualities of narrators in the chain, as well as the text itself.

In the science of hadith, you can have a difference of opinion about the authenticity of a hadith. This comes down to difference of opinion from various scholars about certain narrators in a chain. Some may say "his memory is weak, da'eef hadith" while others say "his memory is good, saheeh hadith." In this case, we clearly understand how and why a hadith can have ikhtilaf about authenticity.

So what about Ijmaa?