NLP techniques to label unlabeled data in a dataset
This is still a problem undergoing active research. I think it's commonly referred to as unsupervised labeling. If the dataset is not too large, it's probably more accurate and time efficient to either do it by hand, or hire someone on Mechanical Turk to do it.

If you're dead set on doing it automatically, trying some form of semi-supervised learning where you train it on a few examples, and then attempt to generalize is probably your best bet.