The most common symptom of a malware infection is a slow-running computer. Are your operating systems and programs taking a while to startup? Is your data bandwidth suspiciously slow? If so, your computer may have a virus.
However, before immediately assuming that your computer has a virus, you should check for other causes for your computer to slow down. Check to see if you are running out of RAM. For Windows, open task manager (Ctrl + Shift + Esc) and go to the Performance tab, and check how many gigabytes of RAM you are using in the Memory section. For Mac OS users, you can open the Activity Monitor application, and under System Memory, you should be able to find out the RAM usage.
Other causes of a slow system include lack of space on your hard drive and damaged hardware. Once you've ruled out the other potential causes, a virus may have infected your device.