If you create a rectangular profile and place a circle within that rectangle then delete any unnecessary segments using the Trim Entities command, the sketch may appear complete and yet moving one corner will move the corner. A circle may no longer stay centered on a centerline, and a solid body created by an extrusion may look correct but later dimension change will move some of the geometry. A sketch that has any of these symptoms is under-constrained.
A simple test that works well is with a small sketch before you begin working on a full part. Try drawing a rectangular profile and a circle then add a width and height dimension to the rectangle then drag the corners, edges and center. Drag the circle and if any geometry moves that geometry has a degree of freedom that has not been removed by a dimension or a geometric constraint.
Dimensions define values like length, diameter, distance and angles. Constraints define relations. Constraints like horizontal keep a line horizontal, coincident joins two points and concentric keeps the centers of two circles in the same location. Other constraints include symmetry, tangent, parallel, and perpendicular. A stable sketch profile typically uses dimensions and constraints; it doesn’t require dozens of dimensions for a simple sketch.
The origin and major axes make good reference points. Rather than dimension all elements against other edges, use the origin, a centerline, or symmetric geometry to define the location of key features. Center a rectangular profile about the origin, place a mirror group of holes at a dimensioned distance from a vertical axis of the workplane. Such sketches are often easier to understand and behave more predictably when you change the size of a part.
Conversely, you can apply too many dimensions to a sketch. If a line has a horizontal constraint and the vertical difference between two endpoints is dimensioned, this may create conflicting information. If you create two concentric circles then dimension the vertical distance of both centers from the workplane, the second may create an over-constrained situation and the software might reject the new relation. You should remove the conflicting constraint rather than deleting random geometry.
As another quick test, sketch a mounting plate with two holes in a simple part. Add dimensions for the plate width and height, add a concentric constraint between the plate profile and a centerline, add the dimension for a hole diameter, and add a symmetric constraint for the holes. Then change the width of the plate and the outside profile will change, while both holes will stay concentric to the profile centerline. If one hole drifts from the centerline or the plate profile moves away from the origin, add an appropriate geometric constraint rather than a dimension.
Your sketch isn’t ready just because it resembles a drawing, but when the geometry behaves as you expect. Before you extrude or cut a sketch, change one main dimension and drag several points in different directions and observe the sketch profile. What you are interested in is whether the sketch responds to the dimension changes you made as you planned it to, not if it stays perfectly still.