There are a lot of ways that bad actors online can compromise your computer. As their techniques become more sophisticated, it becomes more difficult to know whether or not your system has been compromised. There are some signs to look for, however, to tell if your computer is affected by malware.
You can always scan your computer with antivirus software. Microsoft includes its own antivirus utility as part of Windows 8 and above. You can also use the Malicious Software Removal Tool by pressing the Windows Key + R, typing in "MRT," and pressing enter.