Check the date on the poster for historical accuracy. Make sure there really was a concert or event on the date advertised at the venue specified. Visit the library and scour the Internet to find out everything you can about the concert, the number of tickets sold and whether posters were printed for other dates on that tour.
Consider the source of your poster and ask the seller where he got it. If you bought the poster at a garage sale from someone who seems honest, there is a chance that it is authentic. Try to verify the seller’s story about how he came into possession of the poster by talking to other people who were on the scene in the 1960s.
Examine your poster for signs of age. If it is an authentic Beatles poster, chances are it will look and feel old. If it is not torn, rumpled or yellowing, it is probably not 40 to 50 years old.
Consult a collector or auction house if the poster appears to be authentic. Look in phone directories and trade publications to find auction houses; there are dozens of collectors who advertise on the Internet seeking vintage posters.