I believe there's a third party plugin that can show me how emails appear in different email clients - but I'm not sure where/what. Something like Litmus.com, but they're a little pricey for us. Pardot had this included, at no cost I believe.
I'm sure I saw something, but it seems hard to find.
Marketo has an add-on that you can discuss with your rep -- I've never used it and when I reviewed it, it didn't seem that great. There's a cheaper alternative called emailonacid.com also that isn't as pricey as Litmus. Personally I really like Litmus not only for the client rendering but the spam checking also.
It's powered by 250ok. Ask your account rep for more info, it's definitely nice-to-have when you're creating marquee emails.
250ok is essentially litmus. I don't think it is inherently cheaper and it's not actually integrated with Marketo beyond a button.
You are limited to 15 test campaigns/mo, whether or not you use the Design Informant or full Campaign Informant. So it is really great for testing NEW templates or a set of emails you will send often or a big send test.