Thanks for clarifying.
The solution sounds like a mess. And even if i scan for the number of balls on a table it is little too late to announce multiball in optical way. Mostly the table does play some sounds and blah and then multiball is coming. Some tables hide allready some balls on table and release them by different actions.
I thought if i implement the interface "void DataReceive(char TableElementTypeChar, int Number, int Value);" then i could collect some info which can be interpreted as multiball but i guess like you said, it's not possible because no rom send this data.