Mine failed the MOT and they where knackered. They are visible to an extent, with the car on a ramp certainly, or when jacked, by looking through the gap under the triangular plate, more obvious when looking from the direction of the rear dif, see the guide. There is a pic of donut in situ. Press the bump stop/knobly bits around the outside diameter in towards the centre of the bush, if they stay put all is well, if they fold in towards the centre to reveal a rusty mess underneath they are fubar.
As said elsewhere on here there can't be many omegas left with sound original rear donut bushes, my previous car was a 52 plate, bushes failed. Most of the cars done at Newent where facelifts post 01 at least iirc.....and all had a valid MOT.
Failed bushes front and rear, IME, are often missed by MOT inspectors, and I was actually quite pleased when mine where spotted. Tightened up the back end nicely once replaced.
Hth.
Hello
I have same problem, the colder the louder. Where is the guide ? How to change them ?