EDI has to detect an anomaly.

When you go into orbit around a planet, EDI will say that an anomaly has been detected. You then started scanning the planet, following the arrow on the cursor, and send a probe to the white dot that appears. You can then land on the planet and do the mission.

You just hit "A" (or the PC equivalent) to select the planet and if EDI says "I have detected an anomaly" or "anomaly detected" (can't remember which) then you scan for the anomaly and it should then give you the option to land. 
 
Basically, it's brain dead simple (that's not actually meant to be an insult).  The game beats you over the head with pretty much anything you need to know, so you'll never miss a side quest if you visited a planet that contains one.

I don't think you can just look at random planets and find side missions.  You have to be given side-missions that tell you what planet they are on.  Then when you go to the planet, you have to scan it in order to find out where to land.

" I don't think you can just look at random planets and find side missions.  You have to be given side-missions that tell you what planet they are on.  Then when you go to the planet, you have to scan it in order to find out where to land. "