Sharepoint - How do I know if the page is in Edit Mode from JavaScript?

Looks like you're looking for MSOLayout_InDesignMode

var inDesignMode = document.forms[MSOWebPartPageFormName].MSOLayout_InDesignMode.value;

if (inDesignMode == "1")
{
    // page is in edit mode
}
else
{
    // page is in browse mode
}

This will refer to value of the following html input control, which is rendering on the page when it is in edit mode:

<input type="hidden" name="MSOLayout_InDesignMode" id="MSOLayout_InDesignMode" value="1" />

Update: for wiki pages, you will need _wikiPageMode parameter:

var wikiInEditMode = document.forms[MSOWebPartPageFormName]._wikiPageMode.value;
if (wikiInEditMode == "Edit")
{
    // wiki page is in edit mode
}
else
{
    // wiki page is not in edit mode
}

SP.Ribbon.PageState.Handlers.isInEditMode() 

Returns true or false. I use this in publishing pages.

There is also a global object available called 'PageState'. Among the various members of state information is "ViewModeIsEdit". If the value of this member is "1" then you are in edit mode (e.g. PageState.ViewModeIsEdit === "1").

Note: This variable is initialized after sp.ribbon.js gets loaded and is initialized from the _spBodyOnLoadFunctionNames array on body load. So if you plan on using this and need to query a value as soon as the page loads, you will need to do a check to see if the object exists yet.