There are thousands of photography workshops offered every year, ranging from small/local gatherings to megaconventions.
How does one know if a workshop will be worthwhile? How do you decide which events might be of the most educational value?
I try to find out who's speaking and research the speaker; read their blog, flip through pages of their book on Amazon, see what other people say about them.
the best way to decide which workshops you want to go do:
1) find workshops by photographers that impress and inspire you.
2) track down people who've gone to workshops by those people and talk to them. Find out what the workshops are like by people who've gone to them.
3) research online for reports on workshops and read what they say about them.
The second one is crucial. word of mouth is the best advertisement you can get.
I'd agree with what @chuqui said, and add the following:
Determine your goals in attending. are you looking to acquire or improve a specific skill? Are you looking to add to your portfolio? Do you want to meet other photographers?
Read the blog of the presenter, perhaps follow them on twitter or friend them on Facebook. Get a feel for who they are. Compare what they are doing to your goals.
Workshops, including travel, lodging, your time, etc. are a significant commitment and there is a high potential to feel less than satisfied if your goals are not met.
From my experience with the workshops of CreativeLIVE, this is an almost 100% sure bet. And, they're free at the time of the actual recording/broadcasting.