How do you find these? Do you just watch random streams, search for posted clips?
If it's a streamer that I haven't seen before, I'll go through their clips section and skim through any clips with promising thumbnails. 90% of the time, the clips aren't actually labelled (for good reason; the streamer usually deletes them if it's obvious what it is from the title) so there is a bit of luck involved there. If I see anything, or if it looks like there might be something, I'll check the date of the clip then go to twitchtracker.com and find all clips taken on the same day. Twitch usually deletes all VODs a month or two after they happen so I can't always go through the whole video, which is a shame.
When I do find VODs that are still up, I'll hover the thumbnail of the VOD to see what they're wearing, then decide from there. If alilfox, for example, is wearing leggings, then I won't bother, but when she wears jeans there's a good chance that she'll whale tail at some point.
I don't actually watch any full videos/livestreams or anything, I'll just hover the timeline of the video with my mouse to see if the streamer gets up/does something, then go there and see. While you can't usually tell for sure if a whale tail will happen, you can usually tell if there's no chance of it happening when things are too high-waisted so there's not much time wasted.
Hope that helps.