A rough rule of thumb,

when the friction martial is the same thickness as the "brake pad metal backing plate" it time to think of changing the pads.

(because under heavy braking, the brakes can over heat faster,and course brake fade)

I am glad someone else has had higher rear pad wear than front - I think it must be light braking as I do a lot of motorway and at 3rd service, pads were less than 1/2 worn at the back and less than 1/3 at the front.
4mm will get flycop to next service with no problem IMHO Gone... 2013 XS TD4 Manual Firenze Red
Gone... 2008 (09MY) XS Td4 Manual Stornaway Grey
Prior... 2003 G4 Discovery 2, 1998 Disco 1 and 1997 Disco 1 Company car before that
Also gone in Nov'13, after 7.5 yrs, to new owners with time for more TLC - 1980 Series III 88
Current 2015 Discovery Sport HSE 2.0

I get through a set of pads in less than 12000! That's good I think!!!

What's the easiest way to check the brake pads.

What I mean is do I have to remove the wheel to visually inspect or is there a good trick? 2002 Honda VFR800
2002 Toyota Celica 140 Silver (mid life crisis - again!)
2007 FL2 GS Manual Army Reconnaissance Green + freel2.com sticker
2004 Toyota Celica 140 Black - Gone
2000 Toyota Celica 140 Silver - Gone
1998 Toyota Celica ST Pearl Green - Gone
1996 Nissan Primera 1.6 - Gone
1994 Rover Montego 1.6 Auto - Gone