As barcode verification is hard on small ampules when the label is on the product, can verification be done before?

The rule of thumb has always been that verification should always be done with the barcode symbol as close to its final configuration as possible as you want the barcode verifier to “see” the symbol the same way the scanner will.  

If it appears that the verification cannot be done in the final configuration then it is best to consult the verifier manufacturer to determine possible reconfigurations to accommodate.