Some examples of "find out":
1. Boss to employee: "Who was on the phone a moment ago? Can you find out for me?"
2. Teacher to students: "I will find out who wrote those words on the whiteboard!"
3. Traveller: "I need to find out how to reach the station."
I think in your situation, with taxes, you could say:
4. "We want to find out what taxes we need to pay."
5. "We want to find out the taxes payable."
In this instance, you most likely want to know *the types* or *the names* of the taxes.
If you change your questions to:
6. "We want to work out what the taxes are."
7. "We want to work out the taxes."
You are most likely wanting to know *how much* the taxes will be. However, "work out" could be used if you wanted to know the names. This would be if you were reading documents about all of the different taxes and you need to "work out" (determine) what taxes apply.