I've never seen an automated tool, but just turning on Grid Lines in your image editor works pretty good.
If you have a standard size character set, set the grids to 16 x 24. That way you have grid lines around each cell as well as centerlines for each cell.
Use a common feature, like eyes, to align horizontally. Use a common feature, or the bottom of the feet to align vertically. This should result in a
natural looking 'head bob' as the character walks. i.e. the head is one pixel lower when the character is taking a step.