Hi Denis,

as already mentioned in some replies, the best method would be a special account on the target system.

Assuming your clients all use windows servers with active directory structures, your account could be supervised using Local Security Policies. These allow to check AD activities, supervising of GPOs and many more.

However there are some drawbacks here:
Your clients needs an administrator, who is capable of configuring those guidelines and small companies usually don't.
These security settings have to be implemented individually in each customers domain.