Hi OP. Long term on and off depressed person here! I think I've become quite savvy over the years at distinguishing between feeling 'a bit shit' and recognising when I need help with my mental health. That said, I still mess it up sometimes.
For me, the alarm bells ring when I notice that I've lost hope. When I feel like things will never get better, there's no point trying, and I'm paralysed by the pointlessness of it all. This is different to when I feel, for example, like stuff is really hard and I don't know what to do. The latter sucks, but i can usually get through it with better self care, support from loved ones, and few days of annual leave. The former, for me, is ring the doctor territory. It also takes a while to notice, which is another sign, and quite annoying!
That state of hopelessness can look different for different people. I can feel like that on the inside and still function on the outside. For a while. For me, I feel like I'm going through the motions and absolutely nothing gives me any joy, but I can muster myself to get out of bed and go to work (and then sit at my desk for 20 minutes hikding back tears wondering what the fucking point of it all is when I'm only going to fail anyway). Ultimately, if left unchecked, I end up in a mental health crisis that leads to extended time off, unhealthy and unhelpful behaviours, job loss, not being able to look after my family etc. But that has only happened when I've failed to spot the earlier signs.
The other tell tale for me is feeling detached, like I'm observing the world and not participating in it. It's almost visceral, like i'm begind a glass wall.
What I'm trying to say is pay attention to your inner voice and take action early if you need to. Being clinically depressed doesn't always look like sitting at home in a catatonic state and a stained jumper crying all the time. Know yourself, advocate for yourself, and take action before it becomes disabling.
If you have a good GP, a frank chat with them can't hurt. Even if it just helps to sort your own opinions on where you are and what you need.
Good luck OP.