I'm particularly interested in the British, German, Dutch, Spanish and Polish armies. Information on any of them is welcome.

I can't really help with regards to the last three; my knowledge of Spain and the Netherlands in this period is not great, and Poland didn't have an Army. I could, however, recommend some books and leave some pdfs about the former two. For the German Army, I'd recommend Antulio Ecchevarria's book After Clausewitz, which examines German military tactics, doctrine and theory from 1871 to 1914. It also provides some insight into developments outside Germany, which should help. As for the British Army, here's three theses: [Stephen Badsey's] (https://www.repository.cam.ac.uk/bitstream/handle/1810/244878/Badsey12201.pdf?sequence=1&isAllowed=y) about tactics and doctrine in the British cavalry, and [Nick Evan's] (https://kclpure.kcl.ac.uk/portal/files/2934126/543934.pdf) about the development of tactics and doctrine in the British Army from 1897 to 1909.

All of these sources make very rigorous analyses, and forego the usual lambasting of thinkers from the time for not predicting perfectly the First World War. Hopefully this helps!

Well, how one defines 'stack up' can change the answer quite a bit.

The British Army of the period, leading up through 1914, was generally considered to be highly skilled - mostly because it was a professional rather than conscription army - and by the end of the Second Boer War, in 1902, was also among the most experienced European armies. But the British Empire was a naval empire, and their army reflected that. It was an expeditionary force, designed to project power in the far off corners of the Empire, and with the might of the British Navy insuring against an invasion, the BEF was never expected to take on major European power without a coalition of allies (which is why the thought of a Russian march on India terrified Downing Street).

The army of the still very young German Empire was tasked, almost fatalistically, with being prepared for the next war with France - a top contender for the best land army in the world at the time - which meant going to war with Russia at the same time - not thought highly of qualitatively, but far and a way the largest force any power could field - after the 1894 announcement of the Russo-French Alliance. A lot of the legislation that allowed Germany to field what many consider the best army of the Great War came just outside of your timeframe, but the foundation provided by the Prussian military meant that from its inception the German army was a formidable force, most notably in terms of its command and organisation, as a result of the Prussian effort to thoroughly professionalize and meritocracize the officer corp (or at least its chiefs of staff).

Which brings us to the issue of comparison, where I'm just going to use figures from August 1914 for both countries for convenience sake.

So, should the British Army decide, hypothetically to throw the full weight of their peacetime army, including reserves, into an invasion of Germany, they would have approximately 650,000 soldiers in the field to face off against 3,500,000 Germans, where they would promptly be massacred. But, the Germans would have been in the same situation at sea against the British Grand Fleet, or trying to supply and maintain a global empire. But neither was built to be in those situations.

On the Poles: the ones in the German army were well trained, supplied, and led. Their countrymen who ended up outside of the Prussian annexations were resoundingly not.

I don't know enough about U.S. military history to speak confidently bout the reality of their force at the turn of the 20th Century, but I do know that none of the belligerents in World War One thought at all highly of U.S. army when they finally entered the war. Even the Entente powers, who had been lobbying hard for U.S. involvement, tried to use their new ally to take up quiet sectors and noncombat roles, freeing up the battle proven British and French soldiers for combat. President Wilson insisted on independent command and got it, but the fact that it grew to a point of contention is indicative of European conceptions of American martial quality.

Main Sources:

Alexander Watson's Ring of Steel: Germany and Austria-Hungry in WWI

David Stevenson's Cataclysm: The Great War as Political Tragedy