Lucy - There's multiple ways to getting the answer you're looking for.
You could have Lead Source as a hidden field on a form, and pass a value into that hidden field through the URL, such that when someone fills out that form through LinkedIn or Blog, the Lead Source could get tracked accordingly. Say you're directing people to the landing page with URL www.xyz.com, you could use the URL as:
www.xyz.com?source=LinkedIn OR
www.xyz.com?source=Blog
So depending on which URL someone came from, the Source field would get populated with the correct value.
Marketo also automatically tracks which site someone comes to your website from. You can find this information in the 'Original Referrer' field (I love this feature of Marketo!!). It will be interesting for you to see a report with this field on it, to see which sources are driving traffic to your website.
Hope this helps. Thanks!!