What should you use?

You create a custom page layout that contains code-behind.

You need to identify whether the page is in Edit mode or in Display mode.

What should you use?

A.
SPContext.Current.FormContext

B.
SPContext.Current.ListItem.Properties

C.
this.Form

D.
this.Web.ASPXPageIndexMode

Explanation:
MNEMONIC RULE: FormContext

Possible values for SPContext.Current.FormContext.FormMode property are Display, Edit, Invalid, and New.

SPFormContext.FormMode Property
http://msdn.microsoft.com/en-us/library/microsoft.sharepoint.spformcontext_properties.aspx