To say whether or not McCarthy was "right" is a very difficult question due to the nature and scope of his activities. A proper discussion of whether or not he was "right" would involve discussing the ethical norms of a vast array of subjects, such as the extent of freedom of speech, personal political rights, the inherent nature of communism, and the inherent conflict (if any) between communist ideology and democracy; to name a few.

So, instead I will address your secondary question of whether or not any of his concerns were justified. The quick answer is: Yes, absolutely, communist agents were working overtime to infiltrate and subvert American institutions and to steal industrial secrets, and they had a great deal of success doing so.

The U.S. government had a secret program, known to us as the Venona Papers, which involved an ultra-secret operation that ran from 1940 to 1980 aimed at decoding intercepted Soviet communications with agents in America. The program intercepted some 25,000 documents, but was only able to decipher 3,000 of them. From that small portion of documents the government was able to uncover the names of more than 300 Americans working as spies or informants for the Soviet Union, with positions in major corporations, development boards, and government agencies, such as the State and Treasury departments. The Venona project was instrumental in catching high-level Soviet spies such as Julius Rosenberg, who was executed, along with his wife, for giving critical information on the Manhattan Project to the Soviets, and Alger Hiss. The Hiss case is very controversial, but as a senior level State Department employee, it goes to show the level of penetration people were fearing at the time.

The Venona Papers also indicated that the Communist Party of America was actively assisting Soviet spies in their actions against the US. This involvement is why there were so many persecutions of the Communist party in the US. Many people, myself included, would say that McCarthy went to far in oppressing a political party, because political freedom is important, but it would be wrong to say that there was no direct and dangerous connection between the CPA and the KGB.

Whether or not there was any real threat to the US from the Communist party is a subject for debate. But what isn't debatable is that the Soviet Union was actively seeking to infiltrate, subvert, and damage the US as a nation and a political system, and that they had been succeeding in doing so to a great extent throughout much of the Cold War. So in that regard Joseph McCarthy was wise to be afraid of their influence. Whether or not his actions were appropriate or "right" is another story entirely.

If you want to know more about the Venona Papers, and the harm done to the US by these informants, read "Venona: Decoding Soviet Espionage in America" By John Earl Haynes and Harvey Klehr (New Haven [Connecticut] : Yale University Press. 2000)

For a quick investigation, here is a link to a list of Americans exposed by the Venona Papers. (Yes, I know, Wikipedia is evil, but the list is supposedly taken directly from the aforementioned book).