Hey Scarlett,
You cannot mark the particular field, but you can create a custom field called "Complete Lead" (or whatever works for you). Once that is completed, you can create a smart list where any Lead with all the information is filled in is grouped together, and then mark your custom field true. Then, once you have done that, you can create a trigger campaign to watch these fields. You would use the Data Value Changes trigger and add that for every field the Lead needs to be considered complete. Then, you need to add a filter for every field saying that the field is not empty. For ex:
What this will do is every time one of the fields that needs to be filled for the record to be considered completed is modified, they will run through this trigger campaign. When they do and all of the fields are filled, then your flow step would be to Change Data Value of "Complete Lead" to "TRUE".
Depending on how many fields you need to be filled for the Lead to be considered complete, this could take some time to set up, but should accomplish what you are looking to do.
And if you want to monitor Leads that become incomplete, you can create that trigger campaign as well. Use the same triggers, but change the constraints to be New Value is empty. You won't need all of the filters, just the "Complete Lead" filter. Have that be set to "TRUE" in your smart list. Then, in the flow steps, Change Data Value of "Complete Lead" to "FALSE".
That was a lot! Let me know if you need any further clarification.