The most reliable way is to show the sketch-element or the Constraint in the Selection-Window.

SKETCH-ELEMENTS

1.Click the element in the graphic-area

The element shows in the Selection-Window.

2.Right-click the element in the Selection-Window

3.Click Delete in the contextual-menu

CONSTRAINTS.

1.SHIFT+CLICK the sketch-element in the graphic-area

The element AND any constraints you have added to the sketch-element show in the Selection-Window.

2.Right-click the constraint in the Selection-Window

3.Click Delete in the contextual-menu

Note: if a Constraint is added to or between a start-Point, end-Point or center-Point, then you must click the Point, and not the sketch-element.

SPECIAL CASES:

If an element in the Mechanism-Editor is a child to a sketch-element  - e.g. a Profile/Extrusion is a child to a sketch-element  - then you must delete the child element first.

To delete the sketch-element that has a Profile as a child:

1.Close the Part-Editor

2.Delete the Profile

3.Start the Part-Editor

4.Delete the sketch-element

In the Part-Editor

Note: to delete a Polyline, a child to a Gear-Pair or 2D-Cam, you must first, delete the parent Gear-Pair or 2D-Cam in the Mechanism-Editor.

Alternatively, in the Part-Editor

1.Click the element in the graphic-area

The element show in the Selection-Window.

2.Right-click the element in the Selection-Window

3.Click Delete... in the contextual-menu