The basic criterion for distinguishing between capitalists and workers is the consideration of what they own.

Capitalists - own means of production
Wrkers - own their own labor power

Additional criteria:

1. Relation to prevailing mode of production: economic interests, source (not amount) of income.
2. Cultural and moral differences: cultural affinity, way of life, set of social values.
3. Organization into political party.
4. Class consciousness (awareness of common interest, scientific understanding of oppression).
5. Collective interests antagonistic to another social group.
6. Degree of control over life and working conditions.
7. Involvement in class struggle.
8. Production of surplus value.

For Marx, the proletariat is the only revolutionary class:

1. Other classes disappear in the face of modern industry.
2. The petit-bourgeoisie and the peasants etc., try to save themselves from extinction as factions of the middle class. Their actions can be reactionary because they try to roll back history.
3. During revolutionary upheavals of the past, other classes have imposed their own conditions of appropriation on society but the proletarians must abolish their conditions of appropriation to achieve liberation. The proletarians have nothing of their own to secure and fortify; they destroy all modes of private appropriation. "The proletarians cannot become masters of the productive forces of society, except by abolishing their own previous mode of appropriation, and thereby also ever other previous mode of appropriation."
4. The working class is the numerical majority.
5. The working class is the lowest strata: its rising up will spring all above it into the air.
6. The pauperization of the proletariat will make it evident that the bourgeoisie is unfit to rule.
7. The advance of industry replaces the isolation of workers, due to competition, with their revolutionary combination, due to association.

From Bertell Ollman "Marx's Use of ‘Class'," The American Journal of Sociology