When it comes to armour, the answer is...yeah, kinda. Especially in the 15th century the soldiers of different European countries had different aesthetic and practical preferences in their armour. These differences in style include:

• differences in how armour was worn (with a full mail shirt underneath the plate or with smaller mail garments, for instance)

• Differences in how it was decorated (for example, with fluting and file-work,

• Differences in how the armour was constructed (for instance, if the arm armour was all attached together at the elbow or if the lower arm armour, elbow and upper arm armour were separate).

• Differences in what types of armour were worn - for instance the Italian knight's preference for armets through much of the 15th century, or the English archers' slowness to adopt plate armour in the 15th century when continental European infantry were increasingly using breastplates and arm armour.

These differences reflected the style of their nation, and the way they fought. While I am not a student of high medieval armour as much as late medieval armour, in general the stylistic differences between European nations probably become -greater- in the later middle ages rather than less, and then they decrease somewhat as the 16th century progresses, and especially into the 17th century. There are some stylistic features that persist for decades or even over a century, like the German preference for three-piece arm armour (vambrace) construction. But many other things change over time, so that generalizations about what a 'nations style' are difficult. The armour of 1400 looks rather different from the armour of 1500. Decorative styles, helmet preferences and construction details all change, even within the same country, sometimes decade-to-decade. A particularly dramatic shift occurs in southern Germany where in around 15 years armourers shift from the particular 'gothic' style to a 'renaissance' style that is very similar to their Italian rivals on the other side of the alps.

And then there is the fact that armour was an international trade. One of the greatest suppliers of armour to Europe was the city of Milan. So the styles of armour produced in Milan are influential. But Milanese smiths were sensitive to local preferences, and would adapt the armour they made to fit local preferences - for instance making armours that used a great bascinet or a sallet rather than an armet. Italian armours also imitated decorative practices like fluting and construction details like riveted plackarts (the lower breastplate in two-part 15th century breastplates). So often you had foreign armourers catering to local preferences when making their armours for export. Moreover, nations bordering one another (like England and their neighbors across the North Sea in the Low Countries) would often borrow ideas from each other, and knights in one country might comission armours from a neighboring country (or get it all the way from Milan!).

So, back to the original question, would you be able to tell an 'English' man at arms from, say, a French one in 1450? Kind of. The english man at arms might wear a great bascinet and his french opponent might wear a sallet (though we see both French and English knights wear both, the French were quicker to adopt the sallet). The English man at arms might have distinctive pauldrons, sabatons that overlap in the opposite direction, a longer fauld (skirt) to his armour, and fully enclosed cuisses (thigh armour). He armour might be decorated with gilt brass borders, and he might wear distinctive guantlets 'In the English fashion'. But this would not be a 100% means of identification, since cultural exchange meant that no style is 100% pure, and it might be that both English and French men at arms were dressed in a similar, largely 'Italian' fashion because they bought their armour from the same Milanese exporter. Similarly, English archers might be more likely to be armed with cloth jacks rather than breastplates when compared with say, Italian infantryman.

I write a lot more about the diversity of armour styles here and export markets and personal preference here. I address changes in armour fashion here.