I wouldn't worry too much about the idea of knowing whether a premise is true or false. As far as we are concerned, always accept premises as true. It has occurred an exceedingly small number of times in LSAT history that a question is weakened by pointing out an issue with a premise.
I can say absolutely anything I want in an argument (true or not), but so long as the logic is correct, then the argument is valid.
E.g. If I feel like I am 90 years, then I am 90 years old. If I am 90 years old, I deserve the senior citizen discount.
The logic in this argument is as follows:
Feel 90 years - > Actually am 90 years old - > Deserve senior citizen discount
So, if you feel 90, you deserve the senior citizen discount. This is a valid argument. However, the argument isn't sound because we all know that feeling a certain age doesn't mean that you are that age.
So just take the premises as they are (don't try to prove them wrong), and just be prepared in that very rare, one off scenario, that maybe an answer choice will point out an error in the premise. As opposed to pointing out the error in getting from premise to conclusion (which is most common).
I hope this helps. Please let me know if you have any other questions.