How to validate scss
There is a Ruby wrapper around the W3C CSS Validator so you can write a simple script to check the output css from scss.
I also found this integration with Compass so if you use Compass to compile your scss, it might be helpful. I haven't used this so I'm not sure how it works but I've had a good experience with the W3C CSS Validator.
I'd really like the same thing built into rails or my development environment somehow.
I found this online tool to be incredibly helpful (it finds errors, but also hints at things I was doing that could cause unintended results)
http://beautifytools.com/scss-compiler.php
Copy paste .scss file into the left hand side and click 'Compile SCSS'
I would suggest, if you want a validator that's built into your NodeJS development (there may be others for different languages too, but this is what I found),scss-validator: https://www.npmjs.com/package/scss-validator