Well rocking the steering wheel side to side with the engine off and the car stationary will give you a knocking sound and feeling if there is free play in the system.
Then getting as far under the car while it is still on its wheels and listening while the steering wheel is again rocked side to side and making noising will let you listen and feel if it is free play on the outer parts, ie TRE or free play on the inner joint, holding each end on each side in turn will narrow it down, if it is the inner joint then I'd be changing both sides right now and considering fitting new TREs at the same time as buying complete steering arms including TRE is a lot cheaper than buying track rods and then TREs - check out ECP website for that info.