Look at the 'person's' name. When looking at your mail, look closely at the person or robot's 'name'. Most spam is named with a first name and a last name, i.e. Morgan Freeman, Taylor McJessie. Do you recognize the name? If not, it's probably spam. Remember that most message users use nicknames.
Look at the time it was sent. Most spam is sent between midnight and 4:00am.
Look at the file size. Almost all spam will be 3KB. Most spam will contain a small sentence like "Hey, I'm Victoria, wanna date," and include a link to a webpage. At the end of the link, will be a mix of random letters. Do not click the link.
If you've done the above three things and still aren't sure, click on the message. Clicking on the message will not harm anything, but if there's a link, do not click it!