Reading about the military operations of WW2, it is possible to read here and there that the Germans seemed "surprised" by the quality of Soviet tanks like the KV-1 and T-34, and how their infantry and tanks were ill-equipped to fight French and British armored vehicles, etc.
France, Germany and the UK being close neighbours, how was it possible for the Germans to be surprised by, for example, the French arsenal? Tanks like the B1 bis, the Somua S35 or the Matildas have been around for a few years before the start of the war in Europe and France had conscription, supposedly making some technical specifications fairly accessible to the general population. But the standard 37mm gun of the German forces was outmatched in its anti-tank role during the campaign of France, Rommel having to rely several times on his artillery and 88mm AA guns to fight off enemy tank formations.
Hence my question, how informed were the Germans, or the French or the British about their neighbours' weapons specifications before the start of the war? I understand very well that the Germans couldn't steal a B1 Bis the same way all parties were eager to recover enemies' planes or tanks from the battlefield to study and test upon during the war. That the Americans didn't know much about Japanese warplanes, I can admit. But I would find it odd that the Germans or the French were utterly oblivious of each other's respective tanks technical specifications in the months leading to the war.
I think the answer here, is, unhelpfully, "It depends".
Certain systems are easier to hide than others - for example, a publically available book - Jane's Handbook of Fighting ships published fairly detailed full-page spreads of various warships of the major navies. While, it wouldn't have gone into detail about precise armour schemes, it had broad armour thicknesses, armament, and speed - enough so you know what ships are needed.
Things such as paper orders of battle would also have been fairly easy public knowledge, though more detailed breakdowns of the actual readiness/training of each of the units may have been harder to come by.
Other systems are a bit more circumspect - e.g. the Allies knew about the existence of Enigma, and were able to reverse engineer how an early version of this worked (the Poles developed a "Bombe" that was able to decrypt early engima codes). But for truly detailed technical specs, the sides would end up resorting to acquiring samples (e.g. Operation Biting in 1942 - done specifically by the British to steal german radar technology)
Both sides actually knew at least something about what sort of weapons they'd be facing. The Germans had been fighting in the Spanish Civil War, where they encountered various Soviet tanks, and were well aware of the inadequacies of their 37mm gun in facing heavier tanks. The 5cm Pak38 was already in development even before the start of the war, and the Pak40 started not longer after because of German intelligence on Allied Tank capabilities.
They still ended up with inadequate equipment to face their opponents' weapons because neither side was truly read for war in 1939 - Anglo-French rearmament had only truly begun in 1939, and Hitlerwasn't expecting the British and French to declare war on him when he invaded Poland. Weapons can have long lead times from recognizing a need for them to actually seeing service so something ordered in 1938-9 might not be fully ready until a few years later, and then you have to manufacture them and issue them.