Only from context. If there is no context a speaker would add on/ona/ono or make ę audible. With duolingo listening exercises, every time a sentence like that comes up please report it , as it is impossible to hear the difference.

Instrumental. If you're interested in something, that's Instrumental.

Interesuję się psychologią i lingwistyką/językoznawstwem.

Don't forget that there are no such things as silent "p" in Polish ;)