To check the rumen, first smell the breath - you don't want to be smelling any ketones or sourness. Secondly, watch his left flank for the 'rumen wave' and keep an eye, ear (and nose!) out for farts and belches. The rumen should contract every 30 seconds or so, and you can see the wave travel along the flank as it does this. You may also often catch the belch, or sometimes fart, that accompanies its conclusion!