You can double digest with any two restriction enzymes, whose recognition sequences are distantly placed and match the size of the fragments with the total size of the vector... If u want to be sure about the frames and differentiate between other pET28 vectors, then its better to go for sequencing.

Kindly check if you do not denature your plasmid DNA before running your gel. As per sequencing, you can go for that if you want to discriminate your vectors

Thank you Vignesh and Benjamin.