How can I know which option is Renegade and which is Paragon?

Yes, as a rule, Paragon options will always be at the top, and Renegade options will always be at the bottom. In some rare cases, the middle option will also be a Paragon or Renegade choice, possibly of less severity, but this is not common.

Keep in mind however, that while Paragon/Renegade options are always positioned at the appropriate corners, dialog options at the corners are not always Paragon or Renegade. While dialog at the corners will typically capture a Paragon or Renegade 'tone', this is not always the case, and there are plenty of conversations with no alignment consequences where the options do not correlate to an alignment per se.