The likelihood is that if your Switch starts to slow down, you’ll notice it quite quickly. While playing something, you can encounter unanticipated frame drops or controller input lag. Perhaps menus are taking longer to load or disappear when you leave them. The system menu on the Switch can have the same menu slowdown.
It’s also possible that the Switch’s performance has just marginally deteriorated, or that it has done so gradually that you are unsure of whether it was always this way. Perform a few quick checks if you think your Switch could be running slowly but aren’t sure. To see if a game performs better or worse, try playing one you haven’t played in a while. Try a game in both portable and docked mode to see if there is a difference in performance.
You can even use the internet, where there are nearly as many gameplay and walkthrough videos as there are memes (actually probably not that many). Launch the game you want to test, then open a YouTube video of the same game to compare results. However, you should also pay attention to whether your Switch and the one in the video have any model differences (such as standard or OLED).