During a football match, the segregation between the home and away fans can be easily noticed. Majority of the seats are occupied by the home fans , while the visiting team's supporters are often seated in a separate zone with arguably not the best of seating arrangements.

I understand that the clubs allocate a certain amount of tickets to be sold to 'away' supporters. However, how exactly do they know if a person is an away fan? What restrictions , if at all, are there on an away fan to prevent them from buying tickets reserved for home fans?