How to check which ASP.NET Version was used for .aspx files

How to check which ASP.NET Version was used for .aspx files

I've got an old website which I have to recreate in PHP instead of ASP.
The website was written in ASP.NET (.aspx files), and in order to display
the website I have to state which ASP.NET version was used. Can I get that
information out of the .aspx files?
Thanks.