Here is what you can do if you are suspicious:
Most of the companies will have a web site and their web sites will have a "Contact us" link. Request the company to confirm the authenticity of the offer letter by providing them details. Most companies will respond within 24 hours because it is in their best interest also to keep their reputation.
Please see below the details:
Email Subject Line: CONGRATULATIONS...APPOINTMENT LETTER//PLEASE VIEW ATTACHED FILE AND FOLLOW INSTRUCTIONS