Some of the best developers I've worked with were quietly convinced they weren't very good.
Not fishing for compliments. Not false modesty. Genuinely convinced that everyone else knew what they were doing and they'd just been lucky so far.
Imposter syndrome is rife in development, and in my experience it hits the better ones hardest. The developers who coast tend to rate themselves just fine. The ones who keep pushing are the ones who can see how much they don't know yet, and that's a very easy thing to mistake for being behind.
The trouble starts when they're growing. A junior gets handed a slightly chunkier task. Then another. Then they're doing what is, by any sensible definition, mid-weight work. Except nobody's told them. So they assume the work is junior-level and they're just finding it hard because they're not very good.
The work went up a level. Their confidence stayed exactly where it was.
What I actually do
Nothing clever, honestly. Three things.
First, I ease people into bigger work rather than dropping it on them with a ceremony. Nobody does their best work while being told it's a test.
Second, when they finish it, I tell them what it was. Not "nice one" or "great job". Something specific: "That's mid-weight work, and you did it without needing a hand." Generic praise slides straight off people with imposter syndrome. Specific evidence is much harder to argue with.
Third, I bring it into 1:1s. We look at what they've been doing over the last few months and line it up against what their level is supposed to look like. More often than not, they've been working above it for a while and had no idea.
That's it. Easing in, naming it after, and joining the dots in 1:1s.
The slightly spicy bit
If your best people don't know they're good, that's on you, not them.
Somebody will tell them eventually. Usually a recruiter, and usually with a pay rise attached. And when a strong developer leaves for a role that sounds suspiciously like the one they were already doing, there's a decent chance nobody in the building ever said it out loud.
I'd much rather they hear it from me first.
Raising the ceiling, not the debt
There's a version of this that goes wrong. The PM becomes the source of everyone's confidence, people start needing the reassurance, and suddenly you've built a team that can't feel good about its work unless you've signed it off. That's not development. That's dependency with nicer branding.
The aim is the opposite. I want people to stop needing me to tell them. Every specific bit of evidence I give them is something they can eventually point to themselves. Over time they start recognising the level of their own work, and my job in that conversation gets smaller.
If it works, they'll outgrow the role. Possibly outgrow me. That's fine. I don't need the credit. Watching someone realise they're better than they thought is one of the best bits of the job, and it costs nothing but a few honest sentences at the right time.
Good PMs give people bigger work. Great PMs make sure they know they're doing it.