To vet or not to vet ... that is normally ALWAYS the question! *grin*
OK, let's take your questions one at a time ...
"I'm not quite sure how far along she is, so I'm wondering if I should take her to the vet to figure it out, because I'd really like to be home when she has her babies. Is there any way they can tell this?"
Unless you know the exact date she was bred, the vet is only going to be guessing. Can you think back and try to put a date to when she may have been bred? If you can, then the gestation period for cats is anywhere from 60 - 65 days or so (give or take a day or two in either direction for individuals). Simply get a calendar and count the days since you think she may have been bred. That is about as accurate as the vet would be by palpating her - most vets do not see pregnant cats on a regular basis and are not as experienced as those vets who choose to cater to breeders in palpating a female to see how many kittens she may be carrying and how far along she may be.
"Also, are there any other things I should be asking my vet information-wise?"
I think it is probably important that you ask him if delivering a litter of kittens is what is best for her - if she is still early enough along in her pregnancy, he may discuss with you the option of spaying her anyway. If she was an indoor/outdoor cat and this was an "ooops" breeding, you may not know as much as you need to know about the sire's health - is HE up to date on shots and free from other health concerns? The answer to that question is probably not if he is a free-roaming intact male. And the possibility of him being sick is all that much higher because he IS a free-roaming intact male. Intact males fight for their territory and bite wounds transmit blood-borne diseases which can be passed to the female during mating. Those illnesses can then transmitted to the kittens and take my word for it, there is absolutely NOTHING worse than allowing a girl to go to term, then just watching the babies get sick and possibly die one right after another - and knowing that you can do nothing about it. It is an absolute emotional nightmare.