How should a good DM test the loyalty of her players?
The GM should ask them if they're down to play their homebrew system that replaces the D20 with a D144
>The best tests of loyalty usually start with kidnapping someone's mother.
Not everyone is on good terms with their mother, and they could be beneficiaries of her life insurance.
But like her or not, no one will fail to at least try to cockblock their mother should she take up post-marital relations with another man besides their paternal father figure, like that time Moleman seduced and tried to marry Aunt May.
This actually hits close to home. My group is pretty much perfect except for the system. I've considered leaving them before over it. But I stayed for the rotating food duties and eventually a true sense of loyalty to friends.
Leave your DM notebook/notes near them, so that they be tempted to read it while you are "on the toilet" or just leave it in one of your player's house after the session ends
Depending on how they roll their characters after it or the next session you will be able to judge if they peeked your notes or not
Also keep trap of their character sheets but allow them to keep them between sessions. You will know of they make changes on their character stars/equipment
Our group consists of 5 guys and 2 girls. We are very honest with each other but sometimes we have guests playing and when that happens, whoever is DMing does all of the above cus we have had "dishonest/unloyal" invited players
Stats slightly change every 2 or 3 sessions. We eventually notice they have more strength/dexterity/etc
Someone suddenly has more HP than what the dmg had written in the DM notebook
Or someone just so happen to have more gold/items than the last session