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?
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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.