why not just ask your friends how they figured out that your site was penalized? you might just get the answer easily. there are various ways of checking for possible penalty. you can see whether your site is indexed or not and at the same time check to see your inner pages show up or not if you search using site:url string. if both check out, then your site is in google index meaning no obvious penalty. however, there could be other penalties like -30 or -50 etc which is hard to detect. you can sign up for webmaster tool account if you dont have one already. add your site under the webmaster tool and wait for few days for the data to start showing up. you will get lots of info there
Do a site: search of your site, if no results are returned, then there's a problem. If there is a problem, it doesn't necessarily mean there's a penalty. It may be a problem with your site. Why do your friends think its penalized? EDIT: Beaten by the sultan :|
If you are penalized you'll see a significant drop in traffic from Google. Have you seen such decrease in traffic ?
Actually, while searching specific keyword on google for search, it's displaying my url at top 1, then is it penalized or not?