Amal Mattu gives us a very simple way to distinguish the type of advanced AV block by looking at the PR intervals.

• PR interval gradually increasing = Mobitz I
• PR interval fixed = Mobitz II
• PR interval appears random = 3rd Degree Block

You may also find this link to LITFL on SVT vs VT most informative.

Dr. Mattu walks us through a three step method to make identifying heart blocks easy. Just ask three qestions
1) What is the atrium doing? (regular, irregular rate?)
2) What is the ventricle doing? (morphology and thus location? rate?)
3) How are the two related? (Look at PR interval, is it regular? long then dropped QRS, longer, longer then drop? or randomly related? 1st, 2nd, 3rd degree blocks).