The assumptions in MANOVA are regarding the covariance matrices. You should test for equality in the covariance matrices across groups. SPSS calculates the M-Box statistic to test that.
Still, there are many issues related to the M-Box statistic. If you don't "pass" the test (that is, you reject the null hypothesis) one criterion, subjective but useful, is to examine the log of the determinants from all the covariance matrices for each group and also for the error matrix. If those numbers are similar there shouldn't be much trouble.