We have decided to use Wordpress for the website of our startup.
How can I be sure whether a given theme can be used for free?
Questions on Web Applications Stack Exchange are expected to relate to web applications within the scope defined in the FAQ. Consider editing the question or leaving comments for improvement if you believe the question can be reworded to fit within the scope. Read more about closed questions here.
Pretty much, any theme you find on the wordpress themes page are available for free. Sometimes theme developers will release a free version of a paid theme with feature limitations. In this case you will probably be notified that you have the option to "upgrade" after you install the theme.
The most foolproof method is just to download the theme and install it. If it installs and you can use it with no problems, and you didn't pay for it, then it's free :) Unless, of course, you're looking for pirated themes, then shame on you.