feditips, (edited ) 
You can see trending posts, hashtags and links on your server by browsing its "Explore" section.
-On website or web app click "Explore" or # on the right of screen
-On official apps click 🔍 magnifying glass at bottom, and leave search box empty
-On third party apps interface will vary, but usually it's called "Trending" and often in settings menu
Lots more info about this including why things trend, how server admins can prevent dubious things trending etc at:
➡️ https://fedi.tips/where-are-the-trending-posts-and-hashtags-on-mastodon/
