Well - a pair is really just 2 birds. A bonded pair is 2 birds who are bonded and may or may not be of opposite sex. If you are wanting a breeding pair, you should ask if they are proven (ie - have already had babies). You have to be careful how you ask questions and listen closely to the answers.
On normal coloured tiels, you can look for the lines under the tail - females have lines running across. You can see these lines in lutinos as well, but they are very, very faint. If the birds are pied, you can not count on this method of sexing them.