I can usually spot them as I scroll through the latest uploads on Mangadex (usually in Mihon). I also sometimes use the advanced search filters to just look at oneshots if the mood strikes (like it did earlier today). This is the link I have bookmarked to only look at the recent oneshots. The advanced search filters also work in the mihon app. Basically, only select the oneshot format, check the “Has available chapters” box, and sort by chapter uploaded at.