I haven't noticed a tool in Marketo that enables this, Litmus is very good for this but you have to have a high subscription package to enable to evaluate in every email send.
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This information is possible it is just rather manual.
The only solution to this I have found is that you can create a smart list of everyone that has opened your email and use the "Add Constraint" feature to add the option "Is Mobile". As with all data this is not always 100% complete but what you can infer is that you would be able to say x number of people definitely used a mobile device, but the number could be much higher for those devices that this did not register on. You can also add additional criteria like devices, platforms, operating systems etc. but as with all these the information is only so complete.
I hope this helps,
You can use the Email Opened/Clicked filter constraint of Device or OS to see just leads that opened.