What gives away a fantasy world as being badly researched?

I’m going to assume you mean badly developed. You can’t really research a fantasy world until it’s complete.

But it’s actually really easy, so I’m going to ruin your life;

Inconsistent lore

Shifting landscapes (unless it’s built into the world)

New and changing races

Language discrepancies

Mary Sue heroes

Unexplained backstories

Inconsistent events

Too much alignment with our current world, unless the fantasy world is based on humanity

Really, everything has to do with how consistent the rules of the world are. Fantasy falls apart when it becomes a sandbox of random events that have no connection and no purpose beyond telling a bunch of individual stories that cannot exist in the same world.