Is there sufficient evidence to be sure one way or the other?
Perhaps not. There's an entire Wikipedia article on the subject which claims various historians are for or against the idea, and presents some evidence from both sides of the argument.
The question in the OP isn't very precise, for example: would or should developing weapons that are capable of being used in an offensive war, during the 1930s, be counted as "evidence" that Stalin "planned" to "attack the West via Germany"?
The same controversy is documented in the article about the Molotov–Ribbentrop Pact:
Some critics of Stalin's policy, such as the popular writer Viktor Suvorov, claim that Stalin's primary motive for signing the Soviet–German non-aggression treaty was his calculation that such a pact could result in a conflict between the capitalist countries of Western Europe. This idea is supported by Albert L. Weeks.[page needed] Claims by Suvorov that Stalin planned to invade Germany in 1941 are debated by historians with, for example, David Glantz opposing such claims, while Mikhail Meltyukhov supports them.
The following claims to be a summary of the consensus position:
While most agree that Stalin made extensive preparations for an eventual war and exploited the military conflict in Europe to his advantage, the assertions that Stalin planned to attack Nazi Germany in the summer of 1941, and that Operation Barbarossa was a preemptive strike by Hitler, are generally discounted.
However the question may be unanswerable due to lack of published evidence:
Asked to what degree the leaders of the Wehrmacht needed to feel threatened by the Soviet military buildup, van Creveld replies "very much" and adds: "In 1941, the Red Army was the largest army in the world. Stalin may, as I said, not have planned to attack Germany in autumn 1941. But it would be hard to believe that he would not have taken the opportunity to stab the Reich in the back sometime." The actual documents of the Central Command of the Red Army, headed by George Zhukov for the last 6 months preceding the Nazi invasion, remain classified in Russia.
Incidentally there were at the time some similar suspicions in the opposite direction:
Hitler's fierce anti-Soviet rhetoric was one of the reasons why the UK and France decided that Soviet participation in the 1938 Munich Conference regarding Czechoslovakia would be both dangerous and useless. The Munich Agreement that followed marked a partial German annexation of Czechoslovakia in late 1938 followed by its complete dissolution in March 1939, which as part of the appeasement of Germany conducted by Chamberlain's and Daladier's cabinets. This policy immediately raised the question of whether the Soviet Union could avoid being next on Hitler's list. The Soviet leadership believed that the West wanted to encourage German aggression in the East and that France and Britain might stay neutral in a war initiated by Germany, hoping that the warring states would wear each other out and put an end to both the Soviet Union and Nazi Germany.