The flex track with no brand might be the old GT track that was sold as it was devoid of any markings from memory, it was only in code 100 track though, so if you use that as a datum then as dthead said try all the other track pieces up to that and if they are all the same height rail then all the pieces are code 100 rail height. If branded under the sleepers it will make a search easier on the internet for more info.
Now if you have various different tracks you are bound to ask this how do I tell what is what metal in the rails. So we will start with brass which is a gold colour almost that is the easy one steel and nickel silver are a bit harder to tell apart, how ever a simple test is to use a magnet if the rail sticks to the magnet it is steel rail, those that do not then are nickel silver rails.
So sort it all out into Brass Steel and Nickel Silver rails.
Now if you have all these different types of rails in that lot I would suggest you sell off the brass and steel rails and keep any nickel silver rails. Here is why, steel rusts and will need plenty of elbow grease to keep the top nice and clean and shiny. Brass corrodes and does the same trick as steel almost in that you need to clean it a lot. Nickel silver rail while it does corrode a bit the corrosion is electrically conductive unlike the rust and brass corrosion. Nickel silver rails are easy to clean as well, but on no account use sandpaper etc to clean the rails at the most a Peco track cleaning block or similar will do the trick to clean the rail tops.
Now if you are planning on using the track for a layout then check all track you are going to use if it looks a bit mangled, warped or twisted in places reject it, track needs to be as flat as possible on a board and if some track has been taken up roughly then it opens up a can of worms when relaid and you try to use it. Little used sidings are a good place to use secondhand good track though.
Really you want to buy new track to build a layout with as you start with a clean slate so to speak. Points and crossings are notorious for damage being done to them taking them up. I speak from experience here as in years past have used the same points and crossings on numerous layouts and always had trouble on all of them except the original layout that used new track. Sure the new track is more expensive but it could save you a lot of heart ache and trouble in the future.