According to an article from Fast Company, competitiveness and outright sabotage are often difficult to tell apart, but the latter is comparatively rare.
Before you label someone as an underminer, make sure they're not just hypercompetitive. If your coworker is overly competitive, then they will want to beat out everyone. If they are actively undermining you, then they want to see you fail in particular.
The two aren't mutually exclusive, but it's still an important distinction.