Here is a list of local trail conditions for Austin. This is updated by the community, not me. Thanks to CrazyT for putting this together and maintaining it. If you want to be a regular contributor to conditions, ping me and we will get you in touch with the right people.
Please remember that Austin trails are NOT rideable if it has recently rained. We're not the Pacific Northwest, if trails have gotten wet they should not be ridden because you can damage them. I know the Mountain Dew commercials show bros ripping up the muddy trails, but trust me those were not filmed here.
If your tires can leave a mark you should not be on the trails.
If you have access to Facebook, there is also a group discussion here if you want more depth about particular trails or conditions.