How to determine whether to bill tenant for repair or pay for it?

[Tomas Calvino]

So I have a tenant that recently called me because the microwave latch broke so the microwave thinks it is open (even though its closed) and won't work. 

In my lease, the tenant is supposed to pay for any repairs that were needed because of his negligence. Any normal wear and tear is covered by us. 

The microwave gets opened and closed constantly so I'm having a hard time figuring out if it was negligence or normal wear and tear. 

How would you decide in this situation or other similar to this?