The verb conjugation for the 1st person plural (nós) is always the same for the present and past tense, except in a few irregular verbs. European Portuguese uses an acute accent (for example, amámos) in the past form to differentiate from the present one (only in verbs ending with -ar though), but Brazilian Portuguese does not.
Context should tell whether it's supposed to mean the past or the present tense, although with a verb like "comer" you can probably guess that it's in the past tense. Just like in English, people rarely say "we eat rice" (unless someone asks "do you guys eat rice?"), the present continuous "we are eating rice" is a lot more common in this case.