Sonic testings good for finding if it is butted or not.You can tell the difference between normal steel and high carbon steel with a spark test.You need to touch the frame somewhere(hidden) with a grinder and watch the shape of the sparks.A knowing eye can tell you in a second what it is.If you have a good eye,you can find spark patterns online.
If you have a big scrapyard near your house,alot of them have a gun they point at the metal and it will tell you EXACTLY what it is.