How would I know if my MacBook is infected with virus?

That is a scam. It's an ad connected to the website. You can find more information about such things here:

Effective defenses against malware and ot… - Apple Community

Especially this section:

• Beware spontaneously appearing, unsolicited popups demanding immediate action. Think before you click.
  • Popup windows are useful and required for some websites, but unsolicited popups are commonly used to deceive people into installing unwanted software they would never intentionally install.
  • Ad-blocking Safari Extensions can help, but none are completely effective, and all of them can cause unexpected behavior.
  • If you encounter a website, text message, email, or phone call alleging your Mac is infested with some ick or that you need to take immediate action lest dire circumstances ensue, it is 100% fraudulent. Ignore it. Read Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams.
  • If you find Safari has frozen or "locked up", leaving you unable to dismiss the page, read Phony "tech support" / "ransomware" popups and web pages for the solution.
  • Any spontaneously appearing dialog insisting that you upgrade your video player right this instant should be summarily ignored. Such popups are commonly associated with sites that promise to deliver "free" movies, music, or other copyrighted content that is not normally "free", but no website that hosts interest-based targeted advertising is completely immune from that threat.

Anyways my problem is when I open my safari there is an ad that pops up and it redirects automatically to a website.

That's weird but what kind of page does it redirect to? Is it an Apple one?

Did you buy this MacBook brand new? Could it be possible you got it second-hand somehow and the previous owner didn't really sign out of his Apple ID or whatever?

Also, if this is a company-owned laptop, maybe it's work-related messages/prompts that you see?

Sorry for the trillion questions haha, it's just that it's always easier to get to the point of what's happening with some additional details.