There was usually little need for such intelligence work. Major doctrine is too important to be kept confidential for starters. Even today, you can just go online and download the field manuals used for US Army doctrine. (The tactical/technical stuff at the lower level is usually at least somewhat restricted). Building programs also tend to be difficult to conceal, and good indicators.

Plus, there was a commonality of the 'profession of arms.' Throughout the 1930s, there was a lot of exchange of officers between the various countries who would eventually find themselves at war with each other. US officers exchanged with German, German officers exchanged with the USSR, and so on. They were all learning from each other, all sides were improving their militaries, in forumlation of theory, at least, even if finances and industry did not necessarily allow them to do so in practice. Further, doctrinal thought would frequently be published in professional publications, available for everyone to read. This, again, happens today.

So, generally speaking, the doctrines were commonly known. Tactical developments, however, being operated at the much smaller level and being far easier to change quickly, often had to be divined in the war itself from the experience of being on the receiving end.

Edit. I should add, however, that just because doctrine is published doesn't necessarily result in doctrine being followed. The archetypal quote "A serious problem in planning against American doctrine is that the Americans do not read their manuals, nor would they feel any obligation to follow their doctrine even if they did read them" may not be entirely correct, but there are examples in practice. For example, the US Army's tank destroyer doctrine was all but never followed in WWII, and the Japanese doctrine for use of armored forces was similarly almost never followed.

You seem to have forgotten to specify the conflict you're asking about. :)