To calculate this kind of reliability, all you do is report the percentage of agreement on the same subject between your raters, or assistants. Split-half reliability is estimated by taking half of your test, instrument, or survey, and analyzing that half as if it were the whole thing. Then, you compare the results of this analysis with your overall analysis. Statistical packages on most computers will calculate these for you, although in graduate school, you'll have to do them by hand and understand that all test statistics are derived from the formula that all observed scores consist of a true score and error score. Usually, a researcher asks a colleague or expert in the field to vouch for the items measuring what they were intended to measure.
Content validity goes back to the ideas of conceptualization and operationalization.
If the researcher has focused in too closely on only one type or narrow dimension of a construct or concept, then it's conceivable that other indicators were overlooked. In such a case, the study lacks content validity. Content validity is making sure you've covered all the conceptual space. There are different ways to estimate it, but one of the most common is a reliability approach where you correlate scores on one domain or dimension of a concept on your pretest with scores on that domain or dimension with the actual test. Another way is to simply look over your inter-item correlations.