I was reading a book in which I saw that the Baron de Steuben had collected strength reports from each brigade of the Continental Army when he was training them in Valley Forge, but it did not go into any detail. How would a strength report be collected? What would it include? For example, would it be that one brigade can move quickly or that they are good at bayonet charges, or something else?
I presume you're asking about his service in the Continental Army of the American Revolution?
This would be numerical strength: how many men are fit for service in each platoon, company, battalion and regiment of the brigade. This will change daily during a campaign, usually downwards, as men fall sick, are injured, or desert. Since the Continental Army was not well-organised before the Baron became Inspector-General, all of those things were likely common, and simply requiring officers to report their units' strength regularly will motivate them to manage things better.