The output of a fully differential amplifier is a differential signal.  You would need a differential component at its output to see the slew rate behavior.  What do you use for simulation?

In SPICE, you could use a differential-to-single-ended component like an ideal VCVS.  This does not alter the slew-rate behavior and, at the same time, allows you to plot the output, which is single ended this time.

Then you are doing the right thing.  Fully differential amplifiers work the same as single ended ones.  The difference is that, in the case of fully differential amplifiers, the differential voltage is carried all the way to the output.  By contrast, in single ended ones, the differential voltage is converted in a single ended voltage.

Both types of amplifiers have to be compensated so that they are stable.  Even the so called uncompensated ones, or decompensated, which are not stable at unity-gain, have a gain-bandwidth product which is constant at higher gains.  Therefore, there is an integrated capacitance which is charged by a voltage controlled current source (gm) which results in a rate of charge.  This rate of charge is the slew-rate.

So, both amplifiers (single ended and differential ended) behave the same from the slew-rate point of view.  Test the differential amplifier the same way as you would the single ended one.  Just make sure you look at the differential output, and you confirmed that you are doing that.

Hope this helps.