Answer:
There might be a variety of ways. The opposite direction from above... If you had hydrogen and oxygen, you could combust/react them together, and then verify that they produced water as a reaction product, although perhaps that would be difficult to collect.
You might able to use it's acid/base properties. Water will act as both a weak acid (proton donor), and a weak base (proton acceptor), with its 3 primary forms: (OH-) (H2O), and (H3O+)
For example... If you had Sodium Methoxide (CH3-ONa) in its pure, powder form, and mixed it with water, you should get:
CH3-ONa + H2O --> CH3-OH + OH-Na+
You could then distill off the Methanol, and demonstrate that you actually recovered liquid methanol.
If you had tritiated water, you could use a scintillation counter to verify that the tritium is actually present in the water.
You could use Infrared, or NMR imaging to view the OH bond.
Calcium Carbide (CaC2) will produce Acetylene Gas (C2H2) when combined with water. The water has to donate the hydrogen for the reaction.