How can you tell whether you are running a official Android program or one modified by your phone maker? So many times , in this forum , I see answers with descriptions of their phone, contacts or messaging application, with options, settings or feature I don't see on my Samsung Galaxy S.

I presume that either I am running a modified version ( made by Samsung ) or the questioner is.

But how can you tell ?

How do you know your application is not the stock application of Android ( btw: is that the correct term ? stock application ? )

If I look at Settings - Applications - Manage applications - Running, I see f.i. Email. Clicking this tells me it's version 2.2.1. Same with Dialer. Contacts on the other hand , shows version 1.0.

But since no manufacturer info is present , I cannot tell whether this is the official Google software, or some other program.

And following that, can you download the stock application from somewhere ? Maybe it's better than the version on your phone ?
You can compare what your phone had with the emulator for your Android version. The emulator is a pristine Android, with no manufacturers modifications.

It is arguable whether Google-branded applications (which are not included in the emulator), are stock apps or not; but AFAIK Google-branded apps are usually not modified by manufacturers since they weren't open source and also Google-branded apps are updated regularly by the Market.