Good call on being careful phishing is rampant with these services. The best practice is to get the onion address from a trusted, detailed review that verifies it. When I was looking for the onion link, I found it confirmed in a comprehensive guide. The review clearly lists both the clearnet and the official onion URL. You can find the verified cryptomixer onion link in this guide. It's the same one I used, and it worked perfectly. The guide also walks through all the features, which helps you understand the settings before you even connect via Tor.