Never thought about this, but it's actually pretty easy to fix.
I created a GutterRenderer that indicates wether an item has been published to at least one, to all, or to none of the publishing targets.
EDIT: Added Click behaviour. When you click the gutter icon, the Publish dialog will be shown for that item.
First I will show you the code that I wrote for this and then I'll show you screenshots of the setup and the result.
Here is the code:
using System.Collections.Generic;
using System.Linq;
using Sitecore;
using Sitecore.Data;
using Sitecore.Data.Items;
using Sitecore.Globalization;
using Sitecore.Shell.Applications.ContentEditor.Gutters;
namespace ParTech.Library.Gutters
{
public class PublicationStatus : GutterRenderer
{
private readonly ID publishingTargetsFolderId = new ID("{D9E44555-02A6-407A-B4FC-96B9026CAADD}");
private readonly ID targetDatabaseFieldId = new ID("{39ECFD90-55D2-49D8-B513-99D15573DE41}");
protected override GutterIconDescriptor GetIconDescriptor(Item item)
{
bool existsInAll = true;
bool existsInOne = false;
// Find the publishing targets item folder
Item publishingTargetsFolder = Context.ContentDatabase.GetItem(publishingTargetsFolderId);
if (publishingTargetsFolder == null)
{
return null;
}
// Retrieve the publishing targets database names
List<string> publishingTargetsDatabases = publishingTargetsFolder.GetChildren()
.Select(x => x[targetDatabaseFieldId])
.ToList();
// Check for item existance in publishing targets
publishingTargetsDatabases.ForEach(delegate(string databaseName)
{
if (Database.GetDatabase(databaseName).GetItem(item.ID) != null)
{
existsInOne = true;
}
else
{
existsInAll = false;
}
});
// Return descriptor with tooltip and icon
string tooltip = Translate.Text("This item has not yet been published");
string icon = "People/16x16/flag_red.png";
if (existsInAll)
{
tooltip = Translate.Text("This item has been published to all targets");
icon = "People/16x16/flag_green.png";
}
else if (existsInOne)
{
tooltip = Translate.Text("This item has been published to at least one target");
icon = "People/16x16/flag_yellow.png";
}
return new GutterIconDescriptor()
{
Icon = icon,
Tooltip = tooltip,
Click = string.Format("item:publish(id={0})", item.ID)
};
}
}
}
And this is how so set it up and how it will look once it's running:
Figure 1: Create a new Gutter item in the Core database:

Figure 2: Switch back to your Master database and activate the Gutter by right-clicking in the gutter area.

Figure 3: The Gutter now indicates the publication status of your items
