"Virus found"

Have a pop up while surfing on safari, with a message "virus found". The message looks real with an "apple store" logo and without thinking for further consequence I press the "scan now" button, after a while I realise that it could be a scam, and press the switch off button. I restart again and download a free version of the malware bytes to scan my macbook pro, I hoop it was not too late. the malware bytes said: no virus found.

How do I know that my macbook pro not infective by virus?