how to find out if computer is infected with a trojan virus?

It's not infected. But you can always run Malwarebytes to confirm.

It's not. Trojan horses and viruses are different types of malware; any popup using that exact term is a scam, and any other message warning of it in macOS will only be legitimate if you've manually started a scanner or uploaded a file.

(242036)

That term is used by scammers to make it sound as bad as possible. Actually, I'm surprised they don't say you have a Trojan virus worm. May as well get all three categories in there. 😐

This is also the most common term used in an email where they claim to have videos of you doing naughty things, and if you don't send some random amount of money to a Bitcoin account, they'll release the video.

But you'll also see this scare tactic on scam web pages, popups from one and in text messages.

Ignore all of them. You have no such thing.

What makes you think you may have a trojan virus on your Mac?