thistles it depends on age and what thistle, and where they are growing. The flowerhead can be pealed to reveal the artichoke heart. This is nearly always really nice with the odd species a tad bitter, however it is fiddly, really fiddly.
Telling thistles apart isnt easy, but isn't totally needed for eating the other bits. A rosette that needs weeding out of cultivated soil, as long as it is before it has started bolting upwards, the leaf stalks are edible. Simply peel all the thorns off and eat the crunchy juicy inner. Those species that have singular tap root have good eating in root. If pulled up when young enough the root is nice raw, it turns stringy pretty quickly once the plant starts growing. Creeping thistle has a bunch of little roots which are too fiddly to eat and full of grit. How stringy they get once they get taller can be due to how much rain they have. Spear thistle is juicest for is longest, both leaf stalks and flower hearts.
Generally in soft fertile soil most thistles have juicy leaf stems and tasty roots, but the same species in dry poor soil would be stringy. Roots are very dependant on conditions as well. Flowerheads I normally give up on as too fiddly and I dont have much patience getting pricked for so little nom nom. Just try what thistles you have in your area.