the best way to tell is by her actions and the changes in her ... um... chest. .. anyways, its funny (in a ironic type of sense), that she is about 6-8 months, and that she seeked you out. often cats who are preggo will become much more caring and usually get taken inside due to this. and they usually find a permanent home during this time as well.
Like you are doing, she needs an non-ending supply of food and water. especially since its unclear how many there are.
a few things is, if her ... chest.. has pockets under them, which would be milk being produced, she will only have 1-4 weeks left to go. since she is a younger cat, keep the vet number on hand incase she goes into labor and has some sort of complications. (make sure the vet has a emergency reachable number). younger because it means shes not fully grown yet, and its unknown how grown the male cat was (or male catS were).
She'll start being restless and doing things that might make a person think she's in a litterbox but it's. generally the girl is just "nesting" which means shes not actually going to be using the potty outside of the actual litterbox (unless its an accident).
give her options during this point for her to choose and decide if it's safe to have her kittens there. make sure she's in an area that no other pets or animals have access to. and also keep moving her away from behind the tv or other areas that are commonly a corded area. (lol seems like the number one place cats like to nest down in, is behind the tv around the cords).

theres two types of preggo cats.. the ones that want to be alone and isolated while giving birth, and the ones that just might squat down right on your lap. but once she does go into labor, you'll want to keep the area calm, and just let her do her thing, because if any sudden sounds or a "oh no! not there!!" happens, she's likely to cease the labor and will put it on hold for anywhere from 2 hours to 4 days (and if it happens theres a more likely hood that there will be complications during labor the next time). depending on the size, and her willingness with wanting you around during this time, she may need help with the first few kittens that come along.
Most of the things online are geared towards the breeding community, but its the same for stray cats as well. the term they use for the preggo cat is the "queen".

my cat that i had, cassie. she was small and young still, but things went smoothly for the most part. I had to help in the beginning because she couldn't reach her back area for the first few kittens, but she did it all herself besides that. and then later on when we thought it was finished, i came back to count the kittens and make sure all were okay, there was one extra.. lol so it was quite a suprise to find out there was 6 kittens instead of 5.

it will be pretty cool towards the end of her pregnancy, because you'll be able to see her belly moving around as if it were pooh's hungry tummy rumbling. and you'll prolly even feel them kicking and punching and rolling if you hold your hand gently on her tummy.

she'll also be licking her behind area a lot beforehand.

one of the main key pics that i have, is this one (she was a few days off from giving birth) :

of course like humans, all cat pregnancies differ.