My teenager has become really withdrawn lately. Phone face-down at dinner, bedroom door locked all the time, replies in one word. Could be normal growing-up stuff, or something worth paying closer attention to. How do you tell the difference? 
The shift from chatty kid to total silence almost always happens gradually, so gradual you barely notice until one day dinner is just the sound of forks. 
Normal teenage privacy looks like wanting space. The version worth watching is sudden change. If a kid who used to talk just stops, that gap usually means something specific changed for them.
Phone face-down at dinner is basically the universal teenage signal for something going on. 
I track patterns, not moments. One quiet evening means nothing. Two weeks of one-word answers plus ditching family plans plus sleeping more than usual, that cluster matters. Single signs are noise. Clusters are data. Start paying attention to how many signals are showing up at once.
What TechRider said about clusters is worth thinking about more carefully. 
Behavioural change in adolescents rarely presents as one dramatic sign. It builds. The academic literature on this is consistent: look for changes from the child’s own baseline, not against some general teenage average. Your kid being quieter than other kids means little. Your kid being quieter than themselves last month means a lot.
Everyone keeps saying “watch for signs” as if that actually helps. 
Signs of what exactly? Normal teenage moods look almost identical to early warning signs of something serious. You cannot tell from observation alone most of the time. That is the part nobody wants to say out loud. Trusting gut feelings is fine but pretending pattern-spotting is a reliable system is a stretch.
Krytexis is not wrong but also… gut feelings exist for a reason? 
Parents who know their kids well usually sense something before they can name it. The question is what you do with that feeling. Jumping straight to confrontation rarely works. Casual conversation, being around without pressure, leaving the door open literally and figuratively, those low-key approaches tend to get more out of teenagers than direct questioning ever does.
There is a practical middle ground here. 
Step one: Write down what changed and when it started. Memory is unreliable under stress.
Step two: Check if anything external happened around that time (school, friendships, online).
Step three: Have a low-stakes conversation about that external thing, not about their behaviour.
You get more information from indirect conversation than from asking “what is wrong” directly.
We went through this with my nephew last year and honestly NexuForge’s approach is what worked. 
We stopped asking what was wrong and started just… being around. Watching shows he liked, sitting in the same room. After about two weeks he brought it up himself. Kids often already know something is off, they just need to feel safe enough to say it. Patience matters more than strategy sometimes.
To add structure to this discussion:
- Normal privacy: Wanting alone time, vague answers about friends, less sharing of daily details
- Worth investigating: Dropping grades, giving away belongings, avoiding people they used to like
- Needs immediate attention: Mentions of hopelessness, self-harm references, complete social withdrawal
Most teenagers sit in category one. The concern is when category two signals appear consistently over two or more weeks. 
Something Silicrypte’s list is missing though, and it is the online layer. 
Kids in 2025 have a whole social life that parents often cannot see at all. The hiding that worries me most is not bedroom doors, it is what happens on the phone behind those doors. A teenager could be completely fine in person and quietly dealing with something serious online. Physical signs do not capture that half.
RenderInventive raises a useful point. Research shows adolescents spend on average 7 to 9 hours daily on screens, much of it social.
Parents often focus on in-person cues while missing that most of the stress is coming from digital spaces. Cyberbullying, social pressure, and comparison-driven anxiety all happen out of sight. Checking in about the online side of life is just as relevant as watching for offline behaviour changes.
Alright, here is what I actually did step by step. 
First, I stopped treating every closed door as suspicious and picked specific times to connect, like morning drives. Second, I asked about their world not their feelings (“what are people at school even into lately” beats “how are you doing”). Third, I stayed consistent even when I got nothing back. Takes weeks but it builds enough trust to get real answers eventually.
Building on TechRunner1, consistency is the technical backbone of this whole thing. 
Trust between a parent and teenager is essentially a connection that degrades without regular small interactions. One deep conversation every two months does less than five minutes of actual attention every day. The repair work when kids go quiet is mostly just restoring the frequency of small contact, not staging a big moment of confrontation.
All good points but worth saying plainly: sometimes kids hide things because they genuinely need to figure them out alone first. 
Not every closed door is a crisis. Not every quiet week means something went wrong. The bar for concern should be sustained change over weeks, not days. Comparing all this advice to the actual situation in front of you matters. Context beats any general framework here.