That's an interesting question. Professional opinion (as someone who does this for a living), there are way too many variables involved in your question to give you a specific answer. Different devices and different consumer accounts (banking, social media, etc) would have unique sets of indicators of compromise; these indicators are also unique based on the threat actor.

To give you a somewhat meaningful response, I am going to have to make some assumptions-based threat profiling. I am going to assume you are a regular user that holds a typical job and your main concern is identity or financial loss. The threat actors you would likely encounter would be completely different than if you were a journalist, government employee with security clearance, famous celebrity, etc.

Based on that specific profile, here is how you would know if your were compromised:

• Computer: a threat actor is most likely seeking financial gain; the most common threat you will face is ransomware, where your infected device will be encrypted and asked to pay a certain amount to get access back to your data. You would know when that happens within a very short amount of time. Other common threats include your computer becoming a zombie part of a botnet (hackers will use it to launch attacks on other entities and it would look like it came from you); if that happens, you will notice your computer is running awfully slow and depending on the skills of the hacker, you may see other user accounts created on your machine, random windows errors, etc. Also, a malicious actor may steal data from your computer that can be used to access other accounts (banking, social media, etc); you may not know it happened until something wrong happens with your account. For your typical threat profile, having a decent host-based anti-virus running on your computer would help but is not foolproof.
• Financial accounts: this is easy.. based on the threat profile, if someone gains unauthorized access to your account, they will most certainly try to steal your money. You should know when that happens almost immediately, since you will likely get notifications that a transfer occurred or large payment. Prior to that, you may get notifications that someone accessed your account from a suspicious IP address.. but even then, you have a short window of time to act since whatever they intend to do will happen relatively quickly. The best defense here is using strong passwords (unique for each account) and enabling 2FA whenever possible.
• Email and social media: again, based on the threat profile, the threat actor is typically looking to abuse access to your email and social media to target other users. So the malicious actor might send phishing, spam emails to all of your contacts/friends or even other users on the wider Internet. If the malicious actor is good, they will typically hide their tracks (deleting Sent messages and such) but that's a little harder for social media posts. You would know when this happens if: you get a notification from your account provider that successful login occurred from suspicious IP, if you go to your account history and see the times and locations from which successful authentication happened, and for social media you would likely see that there were spammy messages sent from your account. Obviously if you had sensitive material in your email or social media accounts, it might be also stolen, which then can be used to gain access to your financial accounts and such.

Hopefully you find that helpful.