Sabers flashing in the sunlight, the bang of pistols, the cry of horses and men. Hussars, chasseurs, chevaulegere... Not quite, but close. Louis XIV had one regiment of hussars, a total of four squadrons. Scouting and screening was done by ordinary cavalry.
There were limits on what generals could do. For example, in some armies patrols had to be from several different regiments so the loss of a large number of men (always a possibility) wouldn't heavily hurt one Colonel or Captain (who had raised the troops). Other generals used dragoons for their scouting, but dragoons were often mounted infantry, and were better suited for outposting/defensive work. Dragoons, though, often had other uses as advanced guards.
There were other, simple methods of conducting scouting. One of the easiest and most common was to develop a uniform plate. This is simply a visual of each enemy regiment. You could show this to a peasant or a town dweller, and they could point to the uniform they saw. That way you soon knew the regiments that were present. Enterprising officers would also scour the letters at the local post office, sometimes learning the names of generals who had passed through.
We don't put individual regiments as counters on the map. Ideally all we have are generals. But there was often enough other information available that people would know: 2) the opposing generals; 2) a rough count of numbers. But there are big differences between "He has 50,000 men" and knowing that it is 42,500 men. So how to translate it?
So: a player can ask his opponent the names of the generals in a stack, though he doesn't have to give their seniority. He does have to give their rank. Second, when asked, a player has to give the strength of a stack within 5,000 men (he can round up or down).
This is crude and simple, but at the beginning of a campaign players have to give each other a copy of their order of battle, though without the strength of each regiment/brigade.
When playees write their accounts of the campaigns and battles, it would not be surprising if they minimized their strength and maximized that of the enemy.