How do you know you reached a plateau

How do you decide you reached a plateau?
Since a reasonable average gain (after the first newbie gains) seems to be about 1 inch/year(?) which is about 2 mm/month it seems hard to actually see that one has reached a plateau until at least 2 month of non-progress has passed?
I am mainly curious about how you decide you reached a plateau and are needing a new routine. I do not think I am close to reaching one yet, but still the idea of going on for 2 months with no progress until deciding to change routine is not a plesant one.
Are you deciding based on measurements only and if so after how long, or are you going by feeling?

Oden

You can consider that you have reached a plateau when you have not gained for 3 months.
The best is a 6 to 8 weeks deconditioning and change your routine when you get back.
I just experienced a huge plateau lately.

Portugesus