The ideas of Karl Marx include both descriptive ones (based on evidence) and prescriptive ones (solutions for the problems he describes). While description is grounded in historical and sociological observation, solutions, as they look to the future, are always speculative, as we cannot have accurate knowledge of the future.
The first major evidence Karl Marx used to build his economic theories was that concerning the treatment of workers in rapidly industrializing nations. In particular, he was influenced by the work of Friedrich Engels who wrote The Condition of the Working Class in England, and later cooperated with Marx on the Communist Manifesto.
Many of Marx's notions about "communism" and the term itself were influenced by his knowledge of evidence about the Paris Commune (1789-1795) established during the French Revolution.