.aspx files, compared to corresponding .aspx.cs code-behind files) are expected to contain minimum programming code, i.e. large chunks of HTML with here and there the calls to variables, eventually with straightforward loops and conditions.
This means that you will rarely find unit tests for .aspx files, since there are no complicated algorithms or business rules there.
Since .aspx files can still introduce bugs, system testing seems a good choice. The way would be the same as the one you use to test whether your HTML code corresponds to the requirements.
Note: ASP.NET encourages to put too much programming code directly in .aspx files. In Microsoft's demos, you can even find SQL queries there. This is a terrible way to develop websites, and is misleading of what templates are.
For example, in Django templates in Python, things are much clearer, and you don't put your database logic in HTML templates. ASP.NET makes separation of concerns not obvious.
If you find yourself putting business logic or database logic in your HTML templates, or if you notice that many bugs are in your templates, consider introducing layers in your application, and keep only minimum code in templates.