Full Idea
We can never devise an experimental situation in which there are no external forces to act on a body. It follows that Newton's First Law could never be demonstrated by means of experiment or observation.
Clarification
The First Law says things continue in their state unless acted on by a force
Gist of Idea
Newton's First Law cannot be demonstrated experimentally, as that needs absence of external forces
Source
Rom Harré (Laws of Nature [1993], 1)
Book Reference
Harré,Rom: 'Laws of Nature' [Duckworth 1993], p.22
A Reaction
It can't be wholly demonstrated, but certain observations conform to it, such as the movement of low friction bodies, or the movements of planetary bodies.