Idena is not a machine checking if machine used by human is not suspicious like others trying to solve that (user-friendly way, bot-friendly too). Idena is doing it from different angle. It requires human to prof his humanity (not user-friendly, sometimes pain in the ass but also not bot friendly). Every node that work for idena (there are 2400 of them currently) prof his humanity to others by doing Turing test (once per epoch - 13 days currently up to 30 in future). In theory, it is not possible to validate more nodes by 1 person because validation window last only 2 min (for short session for 6 tests) and then 30 min (for long session for 20+ tests). In practice people try to validate more nodes but in most cases it ends with very low score on both accounts or even lost identity. IDENA economic model is set in a way that it is more profitable to have 1 good node than 2 shitty so potential cheaters does not have financial motivation to do that. IDENA has also stake mechanism to prevent from account selling and many more interesting aspects but it is not thread about IDENA.
If interrested in IDENA:
https://idena.io/?view=faq
community made stuff:
https://idena.site/
https://idena.today/
https://idena-apps.org/