EuroBureauQSL, the EURAO's QSL Bureaus Global Network, has an XML interface ready to provide simple information to external software and applications about whether an specific callsign can receive cards via this system. The answer is just: Yes or No.
The power of this simple answer, of course, will depend on what each software can do, but it could take decisions in real time as a result of an on-line query.
In human language would be something like answering this question: can the other station receive QSLs via EuroBureauQSL?
Let's see an example asking the system about EA3RKF this way:
The answer will be in this format (XML):
Where we can see that the callsign inquired has a "Y" (corresponding to "Yes") on the <eurao> label, which means it is enabled in the EuroBureauQSL of EURAO.