The Oxford English Dictionary defines peer review as:

'The process by which an academic journal passes a paper submitted for publication to independent experts for comments on its suitability and worth; refereeing.'

How do you know if a journal is peer reviewed?

• Use Ulrichsweb to check if a journal is peer reviewed.indicates the journal is refereed ie peer reviewed.
  

• You can also google the journal name. Most journals that have a peer review process will have this information somewhere on their website. However, put your critical hat on to identify predatory journals.
  

• You can 'refine' or limit your search to peer reviewed journals in some databases eg, CINAHL Plus

• Not every article in a peer reviewed journal has been through the peer review process eg editorials, letters, book reviews.
  

 Quality

• Peer review may not guarantee quality eg the Wakefield article on autism and MMR published in The Lancet; Bolland MJ, Avenell A, Gamble GD, Grey A. Systematic review and statistical analysis of the integrity of 33 randomized controlled trials. Neurology.87(23):2391-402. Read also the follow up editors' note and author response
  

• Look at Retraction watch.
  
  
• Look at Pubpeer for post publication peer review.

• If you are interested read Richard Smith on The trouble with medical journals, or Bob Roehr on Fraud, not error, is why two thirds of biomedical papers are withdrawn.

Remember

Peer review doesn't mean a friend has looked at what you have written and commented on or changed any of the content.