How to tell rarity of old magic cards?
The rarity of some sets cannot be determined only using the cards themselves. Early in Magic's history, it was intended to be part of the "experience" to let the market determine what was rare, and to let rares be surprises.

Over time, it became clear that players wanted more feedback, and the rarity was added to the card.

Your only hope now is to use an external resource. The canonical resource for all cards is the gatherer at http://gatherer.wizards.com which you can use to filter by individual cards, sets, rarity levels, and so forth. Keep in mind that searching for a card at a given rarity might yield strange results if that card was later released at a different rarity.