To monitor your marketo database, look at your leads database and and see how many leads you currently have and know what your database limit is, and how much you want to grow it. One of our biggest mistakes when we started out, we had every contact record sync from SFDC to Marketo, including missing emails and bad emails. So we quickly reached our database limit with useless records.
If you're the administrator, figure out what you want to do with the invalid emails, and inactive records and decide on a period when it's good to purge.
Check the system smart list in the leads database to check on the possible number of duplicate records you might have and merge either in Marketo or from your CRM.
You might also want to create a subscription or preference center, so your leads can decide which emails they want to receive or not.