You can actually control whether they are or not. In the landing page designer, under landing page actions, is an option to edit the page meta tags. Here, you can control whether the page is indexed and/or followed.
I suppose you could search some sample phrases to confirm. Other than that, there's probably an SEO tool out there to confirm the index/follow settings. Maybe something like this: http://www.northcutt.com/tools/free-seo-tools/google-indexed-pages-checker/
Steven, that's a good suggestion. I do that for our regular site, but I didn't even think to do that for the Marketo landing pages. Found it under our other subdomain and it works!