Hey Vik, This would require a message from the receiving email server to tell us that the message went to the spam folder. Receiving email servers do not do this and so Marketo cannot log it.
There are some tools, including Marketo's Email Deliverability Tool, which can give you an idea of how many emails will end up in Spam Folders. How it works is that, with every email send, you will also send emails to a "Seed List". This is a list of emails addresses across all major ESPs that the tool can track. It will see which ESPs send the email to a spam folder and will report that data back to you. This will give you a general idea if a certain email provider, such as gmail or Yahoo, will put your email in the spam folder.