I think there is a way to check how wide a text is using a particular font - but I don't have my Tk books here so I can't check. I might be totally wrong.

There might however be a simpler solution that does what you want. Try aligning your text in the button. By default, Tk will center text (and hence, if the widget is too small, only display the center part). If you tell Tk to put the text on the left, it will show the left part of the text if it gets clipped. Unfortunally, this has the drawback that if the text is smaller than the button, it won't be centered. If that's not what you want, disregard this suggestion.