Is there any markup or meta-tag which I can place in the pages of my website to tell crawlers which pages are either for web or for mobile?
Basically on a main website you should use metatag:
Where http://phone.example.com/page.html is an URL to mobile version of a page user is viewing right now. That's probably the easiest way to solve things. But in general you should use responsive design.
You can get more information on Google Support: