To Test for Randomization When $X _ { \mathrm { i } }$

Multiple Choice

To test for randomization when $X _ { \mathrm { i } }$ is binary,

A) you regress $X _ { \mathrm { i } }$ on all W 's and compute the F -statistic for testing that all the coefficients on the W 's are zero. (The W 's measure characteristics of individuals, and these are not affected by the treatment.)
B) is not possible, since binary variables can only be regressors.
C) requires reordering the observations randomly and re-estimating the model. If the coefficients remain the same, then this is evidence of randomization.
D) requires seeking external validity for your study.