You could present the user with a series of questions that only a human user can answer.
e.g. What colour is the grass?, What is two plus two?, What does a tadpole grow up to be?
Something like that. Another suggestion is to check for javascript and cookies etc. The majority of bots doesn't have it enabled.
Finally, you could do some sort of a memory game. Show 4 images and highlight 3 in a sequence, asking the user to repeat the sequence. Be sure to offer a feature to repeat the sequence.
There are a number of ways, but none of them are fool proof. Markup and styling languages (HTML & CSS) aren't enough to implement these. You will need to use a server-side language, such as PHP, Java, ASP.NET, C# etc.
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