There are a couple of ways you can tell from my experience, but the "email suspended cause" value in your lead record is one of the best places to start.
The email suspended cause field will return a value if you get a soft bounce (spam block) response from your recipient. If you see an IP addresss of 199.15.215.XX or "em-sj-45.mktomail.com" (or a server address similar to this) in the suspended cause it means that your DKIM signing hasn't been set up or something within your DKIM isn't signing your emails.