If you have this relationship:
Clients::__pkClientID = Contracts::_fkClientID
Then you can set up a calculation field in Clients that returns true if any one of the related Contracts records has a specific value in a specific field:
Not IsEmpty ( FilterValues ( List ( Contracts::ContractType ) ; "Contract A" ) )
In FileMaker 12 or Newer, It's also possible to do this with the ExecuteSQL function.
Works great Thanks