How can I find tournaments which are not full?

Keep on scrolling my friend. There seems to not be a filter as of yet. I couldn't find an empty one myself, so I just kept refreshing it. I allowed my location, etc and I got into one quite quickly.

Update: Just keep refreshing the page, as soon as you seen an orange tournament (not just started) don't think don't look, just try to join.

I find the solution and 100% worked for me try this method hope it helps you too: Source:http://www.gammerson.com/2016/07/open-tournament-clash-royale-trick.html

Solution from the link:

• Searching for weird names in the top search box, example:

  "Let me in"

• bad example:

  "open", "free join", ...

Search for "pass" or "password". Some people include the password in the name, that's how i joined.