well, i bang my heel onto it, and if it hurts, it's too hard.
i tend to run xc courses, and if i get back with my feet hurting from the concussion, ditto.
big cracks in the ground are a bit of a giveaway, too.
You also need to know your horse - mine is happy with the top of the ground, doesn't phase him, some hate ground like that. I am rarely the first to do anything, so I look for a bit of cut in the ground and the organisers to have done some improvement works if it is hard - prep on the landings etc. I also think hard and rutty is a million times worse than just hard, which is presumably why many hunters survive all that cantering on the roads!
personally I want to feel give from the ground. The heel bang is indeed a good test normally!! If XC it has to be good the entire way, was going to enter one last year where all bar one field was perfect. This one field had had cattle in over the winter then that really hot easter spell dried out the ground before it could be rolled, it was horrid and I nearly broke my ankle several times just walking over it never mind the fact they expected you to jump a fence in the field, needless to say I didn't compete!
If you can hear your horse's feet stricking the ground when you are riding around it's too hard. I know a lot of people who jump and gallop their horses around on this type of ground, if you are in the next field and can hear something being ridden it is too hard.