OWOW leaves no room for doubt
When we see 2 identical aircraft traveling across an entire sky at the same altitude, but one leaves a long trail which persists and the other either a short trail or no trail at all, we know something is going on. There really is no room for doubt here because there are no variables in the equation.
All of the common passenger jets we now see in our skies use HBT – high-bypass turbofan – engines, so they all rely on the same technology for thrust. Leaving aside the fact that HBT engines do not emit enough moisture to form trails even in ideal conditions (-57 Celsius / 37,000ft), what we see makes no sense when we consider they are all the same engines, so there should be no difference in what we see being emitted by them.