How to find story quests?

In addition to @cloudymusic's excellent answer, if you look in your quest journal (j) you can filter on main story quests and read the text for the last one you completed. There are often hints there as to where to go next.

First, you could try gaining a level or some through non-story quests and FATEs to see if more quests open up for you based on your level. I know that there are quests of a much higher level than 20 so they don't end there.

If you're sure you've missed a quest, you'll have to start searching.

The brute force approach is to go through every city and every region, thoroughly searching, talking to everybody in order to find any missing quests. You should start with the higher level areas, looking especially for places you've never been to, and then move on to lower-level areas. This would be time consuming and not that fun. If you're sure you didn't miss anything in the cities, you could skip re-searching those for now. To make the trip productive, level your other classes while you're at it and swap back to your main when doing interactions.

A simpler approach given that there's no way for us to know exactly which quest you've missed, even if we have a hunch is to search for a guide. Pick a guide, such as this one at gamerscape and go through the list, looking for any quest(s) you may have missed. Checking through a list of 100-200 quests would take less time than searching every nook and cranny.

If you've done every quest in every guide you can find, then you are really left with only a brute force approach.