In jij the quality of the vowel is supposed to change as in English I. But sometimes it's hard to tell whether this happens. In those cases, concentrate on the first vowel. If it's a clear a sound as in ha!, then you are probably dealing with jij. If it's a neutral schwa sound, similar to e in unstressed the before a consonant, then it's je.
Since je is just the careless variant of jij, there is a continuum between perfectly clear pronunciation of jij and perfectly careless je. In the middle one has to take an arbitrary decision, but I don't think this problem exists with the automatic voice.
Correction: The fast automatic voice's pronunciation of jij is (often? always?) so sloppy that it absolutely does sound like je.
The sound of jij en je should be quite distinguishable. The e in "je" is like the French e in "le". The ij is a difficult vowel, in some parts of the Netherlands it is pronounced like English I. We (Dutch people) call that Poldernederlands. It's ok as a starting point, but the proper pronunciation is something you'll have to learn by lots of practice (sorry..) By the way, "ij" and "ei" are pronounced exactly the same.
For anyone confused by all the confusing and incorrect statements, je is unstressed and jij is stressed. (No italics vs. italics.) https://forum.duolingo.com/comment/3734337/Grammar-Jij-vs-Je-Zij-vs-Ze-Wij-vs-We