It is true Stalin had the Red Army in offensive positions rather than defensive, but I don't know of anything that would indicate they were about to attack. In fact trainloads of raw materials were still rolling to the Germans from Russia when the invasion started. In any case, the Red Army was in no condition to invade anybody, as evidenced by their failure in Finland. The Soviet strategy had been to let German and Western allies exhaust each other, but the rapid fall of France was a shock. Stalin clearly didn't believe he was going to be attacked, and suspected Churchill was trying to trick him into going to war with Germany. Churchill writes in his memoirs that he sent Stalin a simple one paragraph telegram a few days before the invasion telling him that the German panzers had been moved to the border. He hoped the brevity of the message would get Stalin's attention. What he didn't know was his ambassador had seen Stalin the day before and spent some time attempting to get him to ally with Britain, which defeated the purpose.
As it turns out, Stalin already knew the Germans had moved into the neutral zone, because they had told him. Their excuse was they needed to move their forces beyond the range of British bombers. Stalin's main concern seemed to be an accidental war with Germany. He ordered his troops not to respond to any “provocations”, but nobody knew exactly what a “provocation” was supposed to be, so in may cases they didn't resist the initial German attacks until it was too late. Stalin himself was shocked and went into a deep depression for weeks following the invasion. I remember Churchill's line about Stalin's actions in his memoirs, “The wicked are not always clever, and dictators are not always right”.
All true, but keep two other things in mind which make the determination a tad more subtle: the Russians had already begun the "invasion" of Germany's sphere of influence
before operation Barbarossa began, with threatening moves both in the Baltic's and against Rumania. Some of that action was permitted by the Nazi-Soviet pact, but it was clear that Germany believed it went further than the protocols allowed.
[And, it seems on the basis of what we now know Ribbentrop-Molotov contained, the German reading of the treaty was right.]
Stalin also had intelligence of the Nazis' intentions from his border spy networks and from Richard Sorge's network within Germany itself.
It's true Stalin decided to ignore those warnings -- or at least, it's true we're told he chose to do so. But clearly, portraying the faithful, betrayed victim was altogether in Soviet interests, and they were far more clever in manipulating western opinion than the Nazi's were, or than we realized at the time.