A simple test you can perform is the Citrate test, Serratia marcensens is Citrate positive while Shigella flexneriis citrate negative.

Also Serratia Marcensens is motile and Shigella flexneri is non motile if you have already done a Hydrogen Sulfide test you would have been able to already determine between the two.

Also Serratia marcensens is VP positive while Shigella flexneri is negative.

Serratia marcensens is lipase positive and Shigella flexneri is lipase negative

Another distinguishing characteristic is that Serratia marcensens grows a distinct red color pigmentation the Shigella flexneri does not have.

(These are all according to Bergeys Manual of Systematic bacteriology)