Here's an idea (this is not my area however): Consider using the difference in resonant frequency. This depends on length ($L$), Young's Modulus ($E$), mass per length ($M/L$, requires mass of straw, likely found from manufacturer or else use e.g. a food scale) and second moment of inertia ($I$, area moment, unique to the geometry, a tube of known inner and outer diameter can be calculated). Copper has $E = 36\times 10^6$ PSI, stainless steel (according to Engineering Toolbox) has $E = 26 \times 10^6$ PSI.
For a tube of inner diameter ID and outer diameter OD, the second moment is
$$ I=\frac{\pi (OD^4 - ID^4)}{64} $$
Note that those are fourth powers of the inner and outer diameter.
For $E$ in PSI (pound-force per square inch), $L$ in inches ($\text{in}$), $I$ in $\text{in}^4$, and $M$ in $\text{lbm}$ (pound-mass), for a simply supported straw (note: length is length from support to support), we can calculate the resonant frequency by (source):
$$ f = \frac{\pi}{2 L^2}\sqrt{\frac{EI}{M/L}} $$
Where $f$ is the frequency in $\text{Hz}$.
The challenge might be supporting the tube properly, maybe fix the ends internally, or with some steel wire (or paperclips). The resonant frequency can (hopefully) be measured by your phone, with an audio spectrum analyzer.
For an example, consider this straw from Amazon. $OD = 0.3125\text{ in}$, assume $ID = 0.3\text{ in}$ for sake of example, $L = 8.5\text{ in}$, and let $M = 0.01\text{ lbm}$ (product net wt, not sure what packaging it has). It's made of food-grade steel (18/10, 303 grade) with $E = 27-29\times 10^6\text{ PSI}$. The second moment is calculated to be $I = 0.00017\text{ in}^4$. Assuming the straw is ideally supported, the stainless steel straw would have a resonant frequency around $43-44\text{ Hz}$, while a copper straw would have a resonant frequency of around $49\text{ Hz}$, assuming I haven't made a mistake in my calculations.