How to differentiate magnetite from metallic iron?
These are the methods that come to mind:


LIBS
XRF   
Streak color. When scratching on unglazed porcelain, magnetite leaves charcoal-black traces, whereas iron leaves shiny steel-gray streaks. Having a powdery mixture this test can be tricky, unless there are reasonably big shards left. Alternatively, one can try to assemble a bulky piece by gluing the powder together with silicate adhesive or cyanoacrylate (as paracetamol suggested in the comments, "rubbing some of the mysterious powder between two shards of porcelain" can also do).   
Compare density of your sample with $\rho(\ce{Fe}) = \pu{7.874 g cm-3}$, $\rho(\ce{Fe3O4}) = \pu{4.9 .. 5.2 g cm-3}$ and, assuming this mechanical mixture consists of iron and magnetite exclusively, calculate the percentage of each phase.


Magnetite in its mineral form often contains $\ce{(Fe,Mg)O}$, $\ce{TiO2}$ and isomorphous impurities of $\ce{Mn}$, rarely $\ce{Cr}$, $\ce{V}$; therefore physical methods of analysis (1, 2) are more preferable. Due to the same reason I'm not suggesting treatment with $\ce{HCl}$ and other methods based on chemical transformations as primary for this analysis.