I usually run a finger along the tread and if that 'bump' within feels lower then the tyre is legal. Then I look for general shape and glance at the wall to see if OK. So took my car in to change the front tyres, near the 2 mm mark, and asked the fitter to just have a look at the back two (front wheel drive car). Tyres at the back manufactured middle of 2009 and not budget but also not high end. Imagine my surprise to find out how in a dangerous state they were. The fitter saw what I couldn't. The tread was good but within the treads were cracks and once deflated and removed from the wheel it was more obvious. One tyre's profile had changed so that it seemed to be leaning towards the car at the top and was disintegrating. I've never come across this before, heard of tyres going bad if vehicle standing for a long time, but I do a minimum of 250 miles a week. I also check air pressure once a week. Anyone come across something like this before? It put the wind up me a bit to think what could have happened, especially on the motorway.
I have seen tyres like yours many times over the years, I found two of my trailer tyres starting to split around the sidewall where the wall joins the tread. When I took them off the rim one of them had tiny holes where I could see daylight through the sidewall. Good job they had tubes in and were only low speed tyres! They looked fine and still had plenty of tread.