There’s a section of the Flash Forum called “Critique My Flash!” It requires registration, so if you’re not in the forum then you can’t see it. If you’d like people to take a look at what you’re writing and provide some feedback, it’s a place to go.
Note that, because it’s on a password protected forum, posting your story there does not count as publication.
It was correctly pointed out to me the other day that I haven’t posted guidelines for it. I’ll be doing that shortly — perhaps over the weekend. Meanwhile, just avoid erotica, graphic sex or violence, unneeded swearing, and so on; post only one or two stories a week to avoid overwhelming the critiquers; and make sure that you’re helping others on the forum as much as you’re being helped.