How to identify spinosaurid tooth or tell them apart from crocodilians?

[AlexamenosWHG]

Hello folks! 

Recently I purchased some teeth. 

Sometimes, things get misidentified. 

 

The big one is supposed to be Spinosaur (no location given but I assume Morocco)

 

The small one is supposed to be Suchomimus

(Again, no location given)

 

 

 

I have seen (and once had one) crocodilian tooth from Morocco and was kinda similar to the spinosaurid. 

How do you tell them apart?