how do I check for malware

Download and install the free trial of Malwarebytes. It is not anti-virus software and is purpose-built to look for adware/malware and remove them. If it doesn't find anything, you can deinstall it from its Help menu.

The operating system has built-in security that prevents viruses and other harmful entities from infesting your Mac.

Avoid downloading software from any location other than the original developer's site, and that will almost eliminate reasons for Malwarebytes.

you may also want to read Effective defenses against malware and other threats - Apple Community. it's not about removing malware, but rather about how to not expose yourself to it in the first place. it's a long read, but it's worth reading every word.