The reason I ask is since there are 600+ fields, is there a place where i could export a list with all fields in my Marketo instance, with their CRM mapping and a column where it could state if it is being used or not.
It may not worth spendingso much time to open each and every field to check for the above mentioned elements.
You can export fields by going to Admin > Field Management > Export Field Names and this would give you a list of fields in Marketo, their labels, and API names, but wouldn't tell you which fields are in use.
Khushboo, from experience, this is probably going to be a long, arduous task -- the first time around -- but hopefully afterwards you can keep up with it on a regular basis. Anytime we discontinue/change use of a field in SFDC I make sure to update anywhere that the field is used, and then hide it so that nobody uses it accidentally and it doesn't muck up our system.
There isn't an easy way to do the initial cleanup other than what Josh and Rajesh have suggested... my only real suggestion here is to try to keep up with it on a regular basis once you've done the hard job of getting it up to date.