The rules are purposely open-ended concerning mundane tasks like tying knots, but sometimes knowing how well a knot was fashioned is important in a dramatic scene when someone is trying to untie a knot or slip out of one. Here's an optional rule for determining the effectiveness of a knot.
The creature who ties the knot makes an Intelligence (Sleight of Hand) check when doing so. The total of the check becomes the DC for an attempt to untie the knot with an Intelligence (Sleight of Hand) check, break their bonds with Strength (Athletics), or to slip out of it with a Dexterity (Acrobatics) check.
This rule intentionally links Sleight of Hand with Intelligence, rather than Dexterity, but arguments can be made for using other ability scores as the basis for the check, depending on the circumstances.