There are many guides and brochures offered throughout London which are free and will tell you what is on along with full details of prices and locations. These booklets can be quite helpful in telling you what's on in London and can even sometimes offer discount vouchers to use when purchasing your tickets.
Another way to find out what's on at the theatre is to go online and visit one of the many ticket agency websites. Information on these sites is usually accurate and up to date. These ticket agency websites will often have a news section with all the latest information on news about the London theatre world. It is also possible to subscribe to their blog or new section to be automatically updated by email every time they post a new story. This is a convenient way to keep up to date and only involves the click of a button when checking your emails to find out the latest news. The ticket agencies are often told well in advance when a new show will be opening or a current one extended and this information will immediately be put on the news section so this is a way of keeping up to date on what's on at the theatre and what will be on in the future.
You can also keep an eye out throughout London at different advertising methods used by the theatres. The free Underground newspapers often have advertisements of what's on at the theatre as well as details of any offers and where to book. The Underground also has a lot of theatre advertising on the walls when going up or down the escalators so be sure to read these to see of any new shows coming to London.
The most reliable way to find out what's on at the theatre would be to visit the theatre itself. The friendly box office staff will be more than happy to help you with any queries, help you choose good seats and even make a booking there direct. If they know they will also be able to tell you what will be showing next in their theatre next and when the previews will be.
So if you want to know what's on at the theatre then there are many ways of finding out so get to it!