How to alert the CRM User that a field needs a value before Converting a Lead

I have a specific field <Product/Family Interest> on our Lead page which is a picklist.  We don't want to make this field required, however, if there is not a value chosen from the picklist before the User Converts this Lead, is there a way I can set a Default Value to alert the User the <Product/Family Interest> field needs to be populated (required) before they can COnvert the Lead.  Again, we only want this to populate before the User Converts the Lead (not qualifies the Lead).  I've seen this done but not sure how to make this work.