Re:
If you find out it's density you will know whether it is aluminium or titanium.
Remove the bolts, then use Archimedes Principle to determine it's volume (submerge in water in a measuring beaker with a scale on the side to see the increase in volume. Or if your only measuring tool is a digital scale submerge in a totally full vessel of any kind, collect the overflowed water then weigh it, each 1g is 1cc volume).
So now you know it's volume.
Weigh it to determine it's weight.
Density = weight / volume
Density of aluminium alloys are typically approx 2750 kg/m3 (+/- a few percent depending on the alloy compound) whereas density of titanium alloys around 4500. That's enough of a difference that your calculated answer if done correctly will clearly indicate whether your stem is alu or ti.
(Correct formula is actually mass not weight but don't worry about this point for the purposes of this experiment).