If one is looking at the question without hindsight, Stalin's suspicions were justified .
Hindsight? Hardly, none was needed.
Lets sum up shall we.
In the summer of 1925 Hitler publishes "Mein Kampf" which lays out in considerable detail what he intends to do in the event he comes to power in Germany. Unlike any other significant European leader/politician he doesn't simply advocate a change in leadership/government in the USSR, he desires to erase Russia as a nation from the map. Exterminate much, if not most, of the population, with the survivor's becoming a ignorant perpetual slave race. Rather more than anything Churchill said or did.
After coming to power in 1933 Hitler proceeds to crush all political opposition, including the Communist Party within Germany. Their leadership, and much of the rank and file, are shipped off to the early Concentration Camps, many to never be seen again. In France and Britain by comparison The Communist Party's are allowed to exist and have representation in the governments of both countries. Indeed Moscow had contact, sometimes indirect control of these groups in Britain and France. Rather a different approach isn't it.
Also in 1933 all forms of media (print/radio/film) come under the control of the Propaganda Ministry. Everything proposed by Mein Kampf and used to get the Nazi Party into power now become amplified by these organ's. Not really needed since there will never be a election again, so no need to keep hyping this, but valuable if your trying to motivate your people see a Eastern Crusade as not only possible or necessary, but desirable. The Nazi Party does not stop there, Education, Sport and even the free time of children has this theme drilled into them. Yes, some media say less than flattering things about the USSR in Britain and France, but by no means all and the Communist's in these countries are not prevented from getting their stories out as a counter.
Not looking good for Mother Russia is it?
Again, beginning in 1933, Hitler undertakes a massive program of re-armament, one predominately focused on land campaign's. This was no secret, it was in all the papers. France had a massive line of defensive works, The Czech's had a not too shabby one to the south, but nothing of significance facing east, and didn't Hitler write and say something about creating a new German Empire in the East?
In 1936 Hitler's Germany helps Franco crush the Soviet supported Republican's.
In early 1938 Hitler swallows in one gulp Austria and gains her military and arms to supplement his own. In late 1938 and early 1939 he aims closer to the Soviet Union by taking control of Czechoslovakia in two big bites, thus adding the famous Skoda arms works to his arsenal, as well as the Czech military's already produced weapons.
This is getting serious!
Finally in the late summer of 1939 Hitler announces his intention to 'adjust borders' with Poland. He is definitely moving East, toward Mother Russia and in the past 'border adjustment' has meant the end of a counties independent existence. The one and only buffer state between the Soviet Union and Nazi Germany is in Hitler's cross hair's. After this Hitler would have a straight shot to Moscow and a chance to make his well known dreams come true. Stalin has one last chance to fight this battle, not on Soviet soil, but on Poland's which eliminates any chance of her getting adventurous against Russia for generations. Stalin has '100 Divisions' to throw into battle, more than Hitler, a chance to gain three allies to split the burden, a chance to fight this war on Polish and German soil.
None of this is hindsight.
Stalin's actions were based neither on logic or reason, but upon uncontrolled paranoia and greed to make a deal with somebody, anybody, willing to satiate his appetite for expansion. Unfortunately millions of Russian's would pay dearly for his greed and lack of foresight, and yes foresight is both real and very different from hindsight.