Since you've mentioned Excel I'll bet dollars to donuts that your tables are not normalized, and that really matters. You might want to check out db normalization if you're not familiar with the term.
Not sure how I'd use a domain function to aggregate over fields if I don't know if they exist or not. Perhaps WGM could elaborate for us. I'm thinking you'd need to write code that makes use of a collection object with field names (one for each table), loop over one of them and attempt to retrieve that item from the other collection. If it's not there, it will raise an error that you trap and add the item to a string or array, depending on what you want to do.
Not the simplest of exercises and definitely not worth the time if the tables are not designed properly in the first place. I suspect there are a LOT of fields in both tables if you can't easily determine if Field1 of table A is in table B. Besides, two tables would normally not have more than one field that might be named the same because the data is related, and ideally they would not have the exact same name anyway. I would bet that these tables need to be broken up into other tables.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.