Measuring crystal drive strength without a current probe
Next time, I should read those application notes a little more carefully :)

The STM32 Crystal Application Note AN2867 offers an alternate method in Section 3.5.2:


  This current can be calculated by measuring the voltage swing at the amplifier input with a low-capacitance oscilloscope probe (no more than 1  pF). The amplifier input current is negligible with respect to the current through C_L1, so we can assume that the current through the crystal is equal to the current flowing through C_L1.


That is, measure the voltage at the non-inverting input to the amplifier (across capacitor C_L1), because the current through the load capacitor is essentially the same as the current through the crystal (since the amplifier is high-impedance).



The drive level can then be estimated as:
$$ DL= \frac{ESR \times \left(\pi f C_{tot} \right)^2 \times \left(V_{pp}\right)^2}{2}$$
where
$$ C_{tot} = C_{L1} + C_{s}/2 + C_{probe} $$
and ESR is from the crystal, Cs is the board stray capacitance, C_probe is the capacitance of the probe leads, and f is the frequency of operation.

I have not been able to verify this method against the current probe measurement, but it gives sensible values for an NXP S32K development kit (i.e. an 8MHz crystal with a measurement of 0.6Vpp gives a power estimate of 6uW).