How can I know that my Gmail is spoofed or sending spams to other people I sent email to them before? I don't have clear signs of that especially my contact list is empty but I'm afraid this may happen later as I clicked on a strange link in a mobile app.
If someone spoofs your Gmail account, there is no possible way for you to know that. That would be like being able to know if I introduce myself to others as Ali Zahy. The receivers would know if they inspected the headers of those emails, though.
You might be able to determine if your actual account was used to send emails by looking at your sent and deleted folders. But if the malicious emails were permanently deleted, there would be no way for you to know. But, again, the proof is on the receiver's side.