Jump as high as you can and land on your heals .That is how we used to test the going.You also learn to hear the sound of hard going .I often canter on seemingly hard tracks but 50 years of hunting have given me aneye for the ground and I steer my horse from side to side avoiding the hard ,the flints and the deep stuff which will wreck a horse quicker than anything
I you're talking about cantering on grass then whether the ground underneath is smooth or rutted and how much grass cover there is has a bearing on how suitable it as well as hardness of ground. If the ground is smooth and free of ruts and divots and there is a decent covering of grass on it then it's a lot less hard work to canter on than slightly softer ground that is uneven and not much grass covering. I'd be more cautious of very slippery (eg dry ground that's suddenly had a load of rain dumped on it) or very deep ground. I'd agree that ground where you leave a slight impression into it would count as "good" but in most areas perfect ground is hard to find! I tend to let my horse be the judge of if it's too hard (he'll shorten his stride and back off) and if we're squelching and it "feels" slippy then it's too wet but I don't tend to have the luxury of walking the routes on foot beforehand.