Plug in the parallel signal cables, but not the power cables.
Connect only the Neutral side of the plug from each generator (to eliminate a floating neutral differential between the two).
Measure the voltage between the plugs of each generator on the Line/Hot. It should be 0VAC (or really close to it) meaning that they have synced phasing. If there is a voltage difference between the two, then they haven't synced and very bad things will happen if they are paralleled.
Shoot, even connecting the paralleling cables could goof up the internals and it would never have parallel capability again.
