Dear Mary: How do I ensure that splitting the bill is fair?

Q. A Belgian couple (a baron and baroness, no less) are regular visitors to the Highlands and I have come to regard them as friends. We always visit good restaurants and go Dutch on the bills. Recently, however, they have taken to bringing over two young men who are a couple and neither of them so much as ‘taks their hand across their pockets’, as we say here.