MOT doesn't check pad thickness. You cannot check my pads without removing the wheels.
This from gov.uk mot-inspection-manual --
1.1.13. Brake linings and pads
Some brake pads have metal wear indicators so that when the pads become excessively worn the metal indicator touches the disc making a squealing sound. Other pads may have a cut, which if worn away indicates that the pad must be replaced.
Defect Category
(a) Brake lining or pad:
(i) worn down to wear indicator Major
(ii) worn below 1.5mm Dangerous
(b) Brake lining or pad contaminated with oil, grease etc. Major
(c) Brake lining or pad missing or incorrectly mounted Dangerous
So Brake pads are checked where visible, and of course nowadays there are small probes with cameras that can be inserted into small areas should they choose to use them.