It is possible to figure out the scale, but creating scales in a map consumes time and effort. This process takes at least one month to install scales in a map.
Your map's scale legend will always be at the bottom. For USGS topographic maps, 1:24,000 is the scale most often used. So, if you don't have a scale, use 1:24000.
here is nice video about topographic maps
[MEDIA=youtube]CoVcRxza8nI[/MEDIA]