The same way you can be sure that any of the hundreds or thousands of other programs installed on your computer are not keylogging and sending your passphrases

The same way you can be sure that any of the hundreds or thousands of other programs installed on your computer are not keylogging and sending your passphrases and plans for world domination to the NSA, Bilderberg Group, and the Russian mob. That is to say, you can't be sure. However, Dropbox is a company in good legal standing, and they have a lot to lose if they offered client side encryption and then leaked the passphrase. Spideroak (definitely) and Backblaze (I think?) already have client software which offers client side encryption. Whether you trust them is up to you.