How to detect image orientation (text)

You could perform a Fast Fourier Transform (FFT) to convert your spatial image to a frequency/angle representation. Then find the angle with the most prominent frequency. It sounds complicated but it's not that hard, it's pretty efficient, and in effect it tests every possible angle at once, instead of being a hard-coded hack that only works for specific angles. Search for a sample implementation with search terms like Numerical Recipes and FFT.

You'd need OCR for that. Rolling your own OCR would be a bit difficult, but there might be library or something out there worth looking into? Also, even with good OCR, it's not a 100% reliable solution.

I wonder if there are some properties of text you could use to help you do this.

For instance based on a quick glance, there are far more vertical lines in text (l,j,k,m,n etc) than horizontal ones so maybe you could start with this.

But even detecting these isn't straightforward, you'd need to use some sort of filter like a Sobel or Prewitt. They both have horizontal and vertical versions, see here for more info.

Of course the vertical/horizontal lines of an excel spreadsheet would be the strongest edges so you'd have to ignore these and look only at the text.

Alternative: Can you not just give the user an easy way to rotate the images, like the arrows in Windows Picture viewer or just show 4 thumbnail previews they can click on. You might need to cache the 4 versions (if you are rotating) so it's quick, but only if speed turns out to be an issue?