Most messages received by e-mail can be identified as spam without even opening them. Some familiar types of spam messages include those regarding mortgages, credit, work from home jobs, one-question surveys, affiliate sites, loan approvals, free merchandise, and prize claims. One look at the subject line will tell you that you did not request this information.
Additionally, if you do not know the sender, it is safer to delete the message as spam than open it for fear of viruses and spyware. Some spam from anonymous senders may even have gibberish in the subject line which will tip you off not to open it.