Tenant claims washer and dryer not working, wants to put his in..

[Jack B.]

Suspicious tenant claim...Conveniently my new tenant of 3+ weeks brought a washer and dryer with them, which are being stored in the garage. Today they called to tell me that the washer that came with the house leaks and the dryer doesn't adequately dry the clothes. How convenient that they happen to have a washer and dryer that they brought with them...which they want to swap out. They don't want me to buy a new washer and dryer, just that they get to use their own machines.

1) I know from past experience that most tenants test landlords in the beginning of a lease to see who has control in the situation. 

2) I'm tempted to go and run a load of laundry through the washer and dryer myself to see if there is any merit to this claim, rather than just letting them do it. Plus this way they learn anytime they complain about anything they are going to get a visit from me. 

3) I don't really care if they swap the machines out, other than the fact that I have to hook them up when they leave (month to month lease). and the fact that I don't want to just give in to every request so that they think I am at their service every day.

4) If the machines do indeed have issues, I will offer to let them swap the machines or swap them out myself.  If they don't I am considering denying the request. I'm sure it won't go over well as they are both Doctors and think the world revolves around them...

Thoughts? How would you handle it? How have you handled it?