I'm trying to evaluate the validity of the SUS responses- I think it's also called internal reliability or Response bias. How is this done for SUS?
When analyzing survey data you need to separate validity from reliability.
- Validity is a measurement of how correct the answer is.
- Reliability is a measure of how consistent the answer is.
Here's a graphical representation of the concept (from Wikipedia):
In general, SUS has been found to be highly reliable and there is some strong support that it produces valid conclusions .
But you are probably more interested in determining if your survey results are valid for your problem. To answer this, my advice is to find some other measure of usability to correlate your results with (this is referred to as "convergent validity"). For instance, you could compare the results of a similar survey or expert evaluation of the interface with the SUS scores. Or you could compare task completion time (or some other quantitative measure of performance) of a user with previously gathered SUS scores, if they correlate then SUS has "predictive validity", which would be pretty convincing.
If you are interested in reliability, I suggest you use Chronbach's alpha. It is a combined measure of all correlations between questions and essentially means how likely is it that respondents answer questions in a similar way. Alternatively, you could rerun the test with the same participants and see if you get similar answers.
 Bangor, A., Kortum, P. T., Miller, J. T.: An Empirical Evaluation of the System Usability Scale. International Journal of Human-Computer Interaction. 24, 574--594 (2008)