Question

how to confirm the presence of magnesium hydroxide in milk of magnesia.

Answer 1

Put a strip of red litmus into the solution. A change in the colour of the strip to blue indicates the solution is basic.
An acid-base neutralization reaction can be carried out and the change in pH is monitored by pH indicator, phenolphthalein. The colour of the milk of magnesia solution would be pink as it is a basic solution. Addition of any acid like HCl, H₂SO₄ in sufficient amount will neutralize the base, change pH to neutral and then acidic. The colour of the solution will change to colourless.
Magnesium burns with a bright white flame and a flame test can determine the cation present.