I've read that potential form of the ru-verb is formed by replacing る with られる, which is exactly the same for the passive form of ru-verbs. How can we tell the passive form and potential apart in this case?
Context is important. With passive verbs you should look for a に before the verb that will mark the person or thing that performs the verb. This is not the same as the subject of the verb.
For example, if you see the short phrase: お兄さんに食べられた。 You can figure out pretty quickly from the に that this is not the potential. The subject of the sentence is an invisible first person pronoun (pick your poison: 私, 僕, 俺). The object is also invisible - let's make it an おにぎり. So the actual sentence looks like this （私が）お兄さんに（おにぎりを）食べられた。In other words: I suffered my older brother eating an onigiri. The に marks the person who performs the actual action, the が marks the subject who suffers, and the を marks the object. Put that into normal English and you get: My brother ate my damn onigiri.
I wrote a basic introduction to the passive form here: http://howtojaponese.com/2008/02/04/embracing-japanese-expression-get-used-to-it-2/
And then expanded on it in a Japan Times article which I link to here: http://howtojaponese.com/2011/04/27/cool-passive-phrase-yarareta/
And never forget that there's no such thing as passive "tense." That's what I used to call it until someone pointed out that tense always means time-related...past, present, future.