The largest group you can queue with right now is 5 people. And yes, 5 people coordinated on vent and actually playing to win can be devastating, but it's still not the same as a full premade. I had a bit of a chuckle the other day when I was in a Codex match where I arrived with the score a 700+ for Guardians and 26 for Defiant (my team) and some guy was raging about premades ruining warfronts so I asked what shard the premade was from and he told me. So I opened the scoreboard and there were TWO guardian players from that shard in the warfront.
It's not easy to tell when you're up against a premade. Sometimes you're just up against a better team. Proper healing can make or break a team. If your team doesn't have it and the other team does, it can make it LOOK like a premade when all it ever amounts to is one team better suited to success than the other.
I have a little game that I like to play when I'm in a melee sort of mood on my rogue. I call it the "Healer Defense Test". I'll stealth up and sneak up on a healer from the other team and lay into them when they've got 2-3 players nearby. If they turn on me and beat me down, 9 times out of 10 we lose. If they stand there and watch or focus on other targets while I kill their healer, we tend to win even if we're short on healers
And I started doing this after I got fed up with going into warfronts where our team had sometimes no healers at all so I made a cleric and starting running Black Garden. And what I found was that any team that had enough players who knew enough about what they were doing to go after me as soon as they identified me as a healer meant I was under a constant state of attack any time I got anywhere near a group of enemy players. And most of the time, the rest of my team would just carry on and let it happen and then whine about not getting healed.
There's more to getting steamrolled than going up against a premade. And it's usually not hard to assemble your own 5-man group and go in as a coordinated team to tip the scales in your favor.