How do I know if my phone has a virus?

First, a reassuring correction: classic self-spreading viruses are genuinely rare on phones, and nearly nonexistent on iPhones, because Apple and Google tightly control their app stores and sandbox apps so they cannot easily infect each other or the system. Most those pop-ups screaming your phone is infected are themselves the scam, fake warnings on shady websites trying to frighten you into installing malware or calling a fake support number. So the alarming browser alert is almost always the actual threat, not evidence of a real infection.

That said, phones can pick up genuinely harmful apps (malware), mostly on Android from sideloaded apps outside the official store, or occasionally sketchy apps that slipped through. Real warning signs include a sudden flood of pop-up ads even outside the browser, unusually fast battery drain and overheating, a spike in data usage, apps you do not remember installing, or the phone behaving erratically. The fixes, in order: delete any recently installed or unfamiliar apps (the usual culprit), restart the phone, clear your browser history and data, and update the operating system. If problems persist, a reputable security app can scan Android, and a factory reset is the nuclear option that clears almost anything. Prevention is easier than cure: install apps only from the official App Store or Google Play, read permissions before granting them, keep the OS updated, and treat any pop-up claiming your phone is infected as the scam it almost certainly is, close the browser tab, do not tap it.