Checking validity of fields when converting Leads

Checking validity of fields when converting Leads

Hello,

When converting a lead to a contact (and Company), we want to check beforehand if some fields are filled. Ideally a script or action should be triggered to check the fields and then continue if all is fine, or display a message and stop the process.

Reasons:
Leads are created with few data at the beginning (often name and email), and with time the profile is augmented. Until it reaches some minimal data, this leads shouldn't be convertable as Contacts.
Contacts are people we are making business with and data have to be quality.
We can't use mandatory fields as we don't know most of these fields at the Lead level.
Also, some colleague are converting leads with most fields empty, which then breaks our quality of our contact list...

That is the reason we would like to have this check at the moment of conversion.

How can we achieve this ? Did you see another way to get data quality for some fields ?

Thanks