David,
Yes, it is the swollen stem as you mentioned but also the leaf's pedicel form and the place it joins the leaf, the structure of the leaf, also young Hedera grow flat unless it is climbing on something.
The juvenile leaves of a Hedera has the same shape as the mature leaves, only fertile stems change form at flowering time but that happens after many years.
The stem is round [and rigid], while Hedera usually has a flaten stem where on one side roots develope in order to hold on a surface.