What are some of the questions that you ask a customer when taking their order? I've to many horror stories about customers "thinking" they told the cake maker the birthday was a surprise.
Are you talking about taking orders over the phone? I'd ask if it's a surprise first, if that's what you're most worried about! Then get the date they need the cake, when they'll pick it up, location if you're going to deliver, basics like flavor, cake size, design, colors, etc.
If you're working from home like I do, then the safest thing is to get them to email you with all of the details that I've listed above. That way you have a record of what they asked you for. I just had one doozy of a mother-daughter team call to order the anniversary cake for the daughter, but the mother did the ordering, I did what she told me she wanted, but then when the daughter came to pick it up, she said that she had told me something else. My suspicion is that the daughter told the mother one thing, then the mother ordered the wrong thing and was trying to blame it on me (they did the same kind of thing for the wedding last year, don't ask...) If you get them to email you at least you can print it out and keep it on hand for reference.