After the discord profile, the easiest search to do, is to type the username into google. Including quote marks forces google to search for the exact username as written.
You may get results for Instagram, Twitter, Facebook and more. You can then click through to them to check the social media. Just be aware that sometimes multiple people use the same username on different platforms. So when you click through you could look for Minecraft content, or look for other indicators that you have the correct account.
Sometimes you might want to search google for a specific site, as sometimes the search on those sites is not great.
For example, try this to search Reddit:
"Username" site:reddit.com
Using "Chushberry" site:reddit.com will search reddit.com for the name Chushberry. Include the quote marks as they will make google search for the name exactly as written. Then you can click through to reddit and then click on the username to read more (see the Reddit guide for more info).
