The technical part
In the
server list there is a column "Info" in which the letters V, S, Q, M and P may appear. An M means that the server admin allows mid-race joins. When you connect to the server, you can join immediately.
If mid-race joins are not allowed, you must wait until the first car has finished the race. (There is no way to do this automatically. You stay in spectate mode until you click "join".)
It would be nice if you could check how far a race has progressed, without connecting to the server, but AFAIK there is no such feature.
The social part
When you join an ongoing race, you should take care not to get in the way of the other drivers. Stop your car at the end of pit lane, watch the track map, and wait until the pack has passed.
When you are considerably slower than the others (> 2 seconds per lap), it is wise to start the race from the back of the grid. You can either
- Click "join" when the countdown starts.
- Park you car in the grass when the race starts, to let the others pass.
It is considered bad form if a driver who has just joined requests a restart (Shift-R) when the race is still going.