Does Blender have a histogram? I'd like to be able to tell where—and by how much—a highlight is blowing out without having to render, go into Photoshop, back into Blender, turn my lamp down, render etc etc. So I'm wondering, is there a way to do this within Blender, or even better, without the need to render first?
Yes, press T in the image viewer to reveal it, among other things.
In addition, clicking on the image shows some useful information about the pixel under the cursor (in particular, the scene referred and display referred RGB values. Plus some fun things such as screen-space XYZ coordinates, where Z is depth).
You will have to render first to use these features, but you can use low-resolution, low sample test renders for fine-tuning.



That said, you might also be interested in utilizing the wide dynamic range of the images cycles spits out to avoid so many small lighting changes.

See 
How to put a Linear Workflow in practice with Cycles and
Render with a wider dynamic range in cycles to produce photorealistic looking images