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What you can do is export your Marketo database and compare that spreadsheet to this new one. You can remove duplicates based on email address and then the remaining leads will be net new.
If you have their email addresses (which hopefully you do if you're going to import them!) you can create a smart list with "email address is" and plug in all the emails from your spreadsheet. Exporting this smart list won't take as long as exporting your entire DB (although that's a great option too) - then use a vlookup like Devraj suggested to compare the two spreadsheets.
This will also give you an immediate sense of numbers. E.g. if your spreadsheet has 500 email addresses and the smart list renders 400, then you have 100 new leads.
This worked perfectly. Thank you!