Signature is like any other work attachment. To make it mandatory, your only option is 'Attachment category' validation. In your case definition, configure 'Required category' check on either stage entry or submit. You can specify same category in Signature property panel. If that signature attachment is not present during form submission, an error is thrown. This works in both online and offline cases.
This is expected to change in future because Signature control will be available in 'attachment-free' mode. This will enable features that other editable controls have like Actions, etc.
In our application when we are using this control, from what I have observed that after putting signature and clicking on accept there is no such property which holds the signature so how can we use a validate rule AND also is there any way to save the signature into DB without the accept button. I tried calling the same activity on submit which gets called on accept but as the button is outside the control it's not getting the parameters