There are several ways to check your website for malware, but I'm just mentioning the couple that I use...
Google Search Console
This used to be Google Webmaster Tools, by the way. Google is pretty quick about noticing malware on your website. Of course, they'll remove it from their search results, but at least they will also tell you about it in the Search Console.
Sucuri Free Website Malware & Security Scanner
This is my favorite... that's why I don't mention any others. There may be better solutions, but Sucuri seems to be pretty comprehensive. If you go straight to their website, though, there is no mention of a free malware scan. You have to either search Google to find it or you can go straight here: https://sitecheck.sucuri.net/