It depends on either the configuration or programmatic change.
Therefore the most reliable way to check the current value is at runtime via code.
See the HttpSessionState.Timeout property; default value is 20 minutes.
You can access this propery in ASP.NET via HttpContext:
this.HttpContext.Session.Timeout // ASP.NET MVC controller
Page.Session.Timeout // ASP.NET Web Forms code-behind
HttpContext.Current.Session.Timeout // Elsewhere