Often you'll be able to tell by the context. Usually (if not always, I can't think of when you wouldn't add it) æble will have "en" or "et" in front of it. Æblet never will since it's the definite article form of "æble".
Hope that made sense.
er → han
isst → spiser
einen → et
Apfel → æble
It's literally a word-for-word correspondence, so it would seem kind of subjective at this stage.
If anything, the Danish might be easier since you don't have to conjugate/inflect for person or for case.
It's really hard to hear the difference between "han spiser et aeble" and "han spiser aeblet". If you press the slow butten, "et" is pronounced very articulated, but in regular speach, there is even barely a "t" in the articulation.