How to find out about auditions for TV productions

I will admit, this is tricky without a top agent. The reason being, most TV series castings use certain casting directors and those casting directors have an agent top five list, each CD having similar or the same list of agents they will use.
From a casting directors point of view it allows them to present trusted actors to the production companies without really having to send a casting breakdown out through the usual channels.
The main TV casting directors are themselves signed to talent agencies, so, most of the time they use actors who are in their own agency.
If you are one of those lucky few who happen to be signed with an agency who is on the list of a big CD, then great! Contact your agent directly (or your agents assistant) and ask for a breakdown of all the projects you have been put forward for. Take note of the production companies (i,e BBC in house, FOX ) and also take note of who is casting those projects.
If your agent is submitting you yet you are not being seen, then it is worth dropping the casting director a personal email, (make sure you tell your agent as some dont like actors doing this)just saying that you are keen to get on their radar and how much you like the TV shows they cast etc, keep this email very short and sweet with a link (no attachments) to your current online showreel.
If you are on with an agency that just doesn't get a look in (90% of us are, so don't worry),my advice would be to get on IMDB PRO as soon as possible, you can see who and what is currently casting, if provides you with the casting directors contact details (or their agents) and the production houses contact details, write directly to them but only if you have a showreel or if they have seen your work.
Be honest, dont brag about what you have done or doing, keep it short and sweet, send a link to your work online and a small image file, dont forget to include your Spotlight link.
The only way to get seen if you are not with their list of agents is to write to them, so, best of luck!