i just did this so i will tell you what we had to know, yours may be different but it can’t hurt to be prepared with extra knowledge.
make sure you know all the names of each vein and which ones would be best in each situation, which one you should use if one blows etc. there will undoubtedly be an identify this vein section. also know everything you can about solutions. (ie-hypo/iso/hypertonic) like why you would use each and in what situation. there should also be some stuff about electrolytes, blood products, and possible reactions. then there should be some case study situations.
also to be a legitimate certification you must have 3 successful sticks observed.