I don't think so.
According to the World Socialist Movement, socialist societies must utilize a system of common ownership in the place of private property. Additionally, the production of resources and labor must belong communally to the society as a whole with each member drawing equally but freely from the communally produced goods. This communal distribution must be democratically controlled as opposed to owned by an individual or group. Because members withdraw and deposit to the communal surplus equally, members of a socialist society own their own labor and production. But this is not the case with the Inca as the elites controlled the means of productions and distribution.
The Inca state and elites garnered their wealth by levying taxes. Villages and towns across the empire were expected to pay tribute to the state. In fact, all subjects of the Inca Empire paid tribute to the state with the notable exception of the Inca elite. Conquered ethnic groups, lower caste Inca, and other marginalized peoples supported the distribution networks with tribute, but were not permitted to withdraw the same amount of goods as their rulers. The elites not only controlled the redistribution networks but also parasitized the system disproportionately.
Also social castes don't really jive with socialism, and class hierarchies existed in all facets of Inca society. And socialist states don't usually conquer civilizations around them.
So IMO the Inca were not socialist. They were something unique.
sources:
Bauer, Brian S. and R. Alan Covey. “Processes of State Formation in the Inca Heartlands (Cuzco, Peru).” American Anthropologist 104, no. 3 (2002): 846-864. Accessed November 23, 2015. http://www.jstor.org/stable/3567261.
Hovde, Brynjolf J. “Socialistic Theories of Imperialism Prior to the Great War.” Journal of Political Economy 36, no. 5 (1928): 569-591. Accessed November 23, 2015. http://www.jstor.org/stable/1822385.
Lawrence, Pieter. “What Socialism Means.” The Socialist Party of Great Britain. Last modified April, 2005. http://www.worldsocialism.org/spgb/socialist-standard/2000s/2005/no-1208-april-2005/what-socialism-means.
Murdock, George Peter. “The Organization of Inca Society.” The Scientific Monthly 38, no. 3 (1934): 231-239. Accessed November 23, 2015. http://www.jstor.org/stable/15641.
Rostworowski, Maria. “The Incas.” In The Inca World: The Development of Pre-Columbian Peru A.D. 1000-1534. Edited by Laura Laurencich Minelli, 143-188. Norman: University of Oklahoma Press, 1999.
“What is Socialism?” World Socialism Movement. Accessed November 23, 2015. http://www.worldsocialism.org/english/what-socialism.
Zuidema, R. T. “Hierarchy and Space in Incaic Social Organization.” Ethnohistory 30, no. 2 (1983): 49-75. Accessed November 23, 2015. http://www.jstor.org/stable/481241.
Depends on your definition of Socialism, of course, but overwhelmingly - no. Only in anti-Socialist (anti-Marxist) discourse Bureaucracy (state administration of economy) is considered sufficient for a state to be Socialist.
Orthodox Anarchist Socialism implies no hierarchy - a taboo Inca clearly violated by having nobility (and castes in general).
Orthodox Marxist-Leninist Socialism (a clarification necessary to distinguish it from many other "true Marxisms" - and it is the one you are speaking about, I'd guess) is more complicated, but the core principle of Socialist state is that it is Socialist because it continuously makes Socialist reforms (which include industrialization, since it is a prerequisite of Communism) under the unrestricted direct democracy (aka Dictatorship of the Proletariat) - Lenin's famous slogan "Communism is Soviet Rule plus Electrification".
While it is often accepted that some level of direct democracy (as it is commonly understood in West) could be compromised if Socialist economic reforms demand it (Soviet dekulakization, for example; or some elements of Central Planning), Inca state clearly had neither direct democracy nor industrialization to speak of. Therefore it is impossible to name it Socialist in ML discourse even in full-on ultra-Stalinist NazBol mode: there are no excuses for its authoritarian excesses.
If I had to put some label on Inca state it would be State Feudalism (a system similar to bureaucratic empires of Asia - in India and China; the one Marx called "Asiatic mode of production").