How can I gauge her interest subtle?

confusedgamer
We are going out soon for a full day to a zoo, and I am planning to ask her on an official date afterwards, but I want to gauge her interest a bit beforehand, subtly. Do you have suggestions on how to do that? Bumping shoulders, light touch on the hand, or other suggestions?

Also, I'm dreading the date question, because it will change the friendship that has and still is developing.
How can I gauge her interest subtle?
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