Usually ArcGIS apps use the API hosted on the CDN. If you look at the source code (in any browser), find the line below and it will tell you the API version.
<script type="text/javascript" src="http://js.arcgis.com/3.8/"></script>
Question
Is it possible from looking at the Firefox firebug to find out what Javascript API a site runs on - I know its ArcGIS Javascript but need to find out what version it is?
La solution
Usually ArcGIS apps use the API hosted on the CDN. If you look at the source code (in any browser), find the line below and it will tell you the API version.
<script type="text/javascript" src="http://js.arcgis.com/3.8/"></script>