There are a number of ways to decide if an email is genuine. Often, people sending phishing emails are not native English speakers, thus there may be basic spelling and grammatical errors. Or the fake website they built is not entirely convincing. Or the URL (i.e. the website address) in the address bar looks suspicious.
All of those problems can be fixed with enough attention to detail. They could hire a freelance proofreader to check for errors and build a more accurate fake website.
In my opinion, the best way to decide if an email is legitimate or not is to ask. Make direct contact with the organisation or person the email is supposedly from, and see if they know about it. Don’t reply to the original email, as the return address often leads back to the scammers. Instead, look up contact details in an online phonebook, or by going directly to an organization’s website.
Not only does this give you an authoritative answer, you are also alerting the organization that’s being targeted, giving them more time to react.
Which emails should you be cautious about? Any that request personal information (e.g. passwords, addresses) or money.