On Recurbate, Look for the videos that have likes, preferably several likes and a 100% like rate with no dislikes. A video with likes and no dislikes is significantly more likely to have a cumshot than an unrated video but sometimes videos with no likes will have cumshots anyway. Still, it's worth focusing more on videos that have likes rather than potentially waste time on unrated videos. And occasionally, you may that someone commented a timestamp of a cumshot under a liked video. Not often, but sometimes.
Also, look at the preview image with the 4x4 image of 16 frames from across the video that appears whenever you click on a video. If they're just sitting there, appearing to do nothing in all the frames, they're probably not cumming in this video, especially if the video is only like 30 minutes long or something so you should probably skip checking that one. I sometimes click on all the videos of a camgirl I'm interested in, even the unrated ones, and I use this method to save time. If the video is an hour long or under and they don't appear to being doing anything interesting in the preview, I don't check the video and move onto the next. If they appear to be masturbating in a bunch of the upper preview squares and then suddenly stop towards the bottom, I will check the video as it means there may be a cumshot in the video. For videos longer than an hour, I look at the preview images to see if there's any masturbation anywhere and check it out if it looks promising.
Other than that, you just have to quickly skim through a video by checking the preview frames as you slide through the video progress bar.