Guess at / guess riddles
What is the difference between guess at and guess riddles?
When do I use guess at riddles and when do I use guess riddles?
Thank you.

Question

Answer

The two are unrelated.

"guess at" means "estimate".
I'm going to guess at how much that bag weights = I'm going to estimate how much that bag weights.

"guess riddles" makes no sense and you need to reword it to get any kind of meaning.
'riddle' can have a few meanings. One meaning is 'puzzle', so you could say "I guessed the riddle incorrectly" = "My response/answer to the puzzle was incorrect"