How do I check if Debug is enabled in web.config

Use:

HttpContext.Current.IsDebuggingEnabled

This property actually looks at the web.config configuration setting. If you look at it using Reflector you will find that it gets the actual ConfigurationSection object using some internal classes.

the following should work

var cfg=(System.Web.Configuration.CompilationSection) ConfigurationManager.GetSection("system.web/compilation");
if (cfg.Debug)
{
...
}

-Edit- I'm aware this doesn't specifically answer the question, as you asked for Web.Config - which immediately suggests a web.app, and is not decided at "run-time", but it does allow a good way to check if it's debug mode.

On another note, you'd not ideally interchanged between debug and release mode on the same app.. -End edit-

How about using conditional compilation??

http://msdn.microsoft.com/en-us/library/aa691099(v=vs.71).aspx

bool isDebuggingEnabled = false

#if debug
   isDebuggingEnabled = true;
#endif

That surely would make the most sense, and doesn't require any specific references?

Just make sure the DEBUG Constant is turned on in your project (See picture)