Question

What additional tests would be needed to establish the exact position of hydrogen?

Answer 1

The question above is incomplete, the full question is given below:
What additional test would be needed to establish the exact position of hydrogen in the activity series of the following elements: magnesium, zinc, lead, copper and silver.

ANSWER
The position of hydrogen on a reactivity series can be determined by its ability to displace oxygen from the oxide of the metal concerned. If hydrogen is more reactive than a metal, it will displace oxygen from the metal oxide and reduce the metal oxide to its metal. If the metal is more reactive than hydrogen, hydrogen will not be able to reduce the metal oxide to its metal.

Answer 2

Hydrogen is a tricky one due because  
 1. It has one electron.
2. Because of it having one electron and or tibial theory it is only half a shell due.

With hydrogen have one electron in its outer shell and it's being half full for it to become stable, it can either gain one electron to have a full shell or it can easily lose an electron, though the shell is at its lowest energy level.
 Under the right condition, Hydrogen can react as a cation and an anion.