transformer waveform
Yes you can, voltage is always across something.

Most scopes have grounded probes so the act of attaching the ground clip of the probe would ground the secondary, but if you use a battery powered scope or a scope with isolated probes you can measure the voltage accross the secondary without grounding it.

Between the output terminals you'll see basically an image of the input voltage scaled down by the transformer. if the output is not grounded the voltage from the secondary to ground will drift under the influence of capacitative coupling to the transformer primary, electrostactic charge, etc...