On Facebook, it's not as straightforward as on other platforms because they don't have an official "trending hashtags" section like TikTok or Twitter. Even so, there are several ways to get a good idea. One is to search for hashtags related to your niche directly in the search bar, when you do this, Facebook shows you groups, posts, and pages where that hashtag is being used, and you can see if it's trending or if hardly anyone is using it.
Another thing that works is to check out large pages or creators in the same industry as yours. Most use hashtags that are performing well for them, so if you see several repeating the same ones, they're probably trending within that niche. You can also use external tools like RiteTag or even Instagram Insights (if you use both platforms), because what's popular on Instagram often ends up doing the same on Facebook.
On Facebook, you have to do a little manual research, but with searches, observing the competition, and using external tools, you can get a pretty clear idea of which hashtags you should use.