Hello there, welcome to the forum
I would be very careful with this situation as it does sound like it could well be a scam. It is pretty difficult to know for sure whether a cat definitely has bengal in it just by looking, especially since it is a cross so will most likely have features from the other breed even if she is telling the truth.
The only thing i can think of that guarantees a cat has at least some bengal in it, are rosettes. Only bengals carry this trait so if the kittens have rosettes, you know they definitely have bengal blood in them. However, thats not to say if they don't have rosettes they definitely aren't part bengal. Bengals can also be spotted or marbled but these are traits that can also be seen in other breeds.
The only other way to prove it really is to see the papers of the full bengal parent but if she is refusing to show you them, that in itself would make me rather suspicious!
If you do decide that they you would like to get them either way, there are some important factors that would apply to any kitten. Is she saying that they are now ready to go at 8 weeks or that you can just choose your kitten now? Because 8 weeks is extremely young and really too early to be separated from the mum and siblings. Kittens should normally be rehomed at 12-13 weeks of age. They should have received vaccinations at 9 weeks and then 12 weeks as well. If they are rehomed before this time, they are extremely susceptable to illnesses as their immune system hasn't fully developed. Considering this, despite being eager to get a kitten i would wait for a better breeder as there would be nothing worse than bringing home a kitten only for it to get ill because it is too young. And that's not even considering the behavioural problems that can also result from homing kittens that are too young.
Also you will need to consider the health of the parents and whether this could have an effect on the kittens. Have they been tested for PK Def / FIV/ FeLV / screened for HCM? Probably unlikely with a crossbreed but it's worth asking so you are fully prepared.
Are you allowed to see the parents? Seeing the supposed full bengal parent would probably be alot easier to identify whether there is a bengal lineage or not.
Do you get to see where the kittens are being kept? Since it is going to be your pet, you really want to ensure the kitten has been socialised properly and not just brought up in a cage. So make sure you interact with the kitten to see how he/she reacts to people.
As Paiviaso has said, going the crossbreed route can often be dangerous as you do get alot of BYBs who don't take as many precautions as pedigree breeders. Especially using websites such as craigslist can attract some rather dodgy customers! However i wouldn't say that all crossbreeds should be avoided by any means...
I would also recommend the rescue / ex breeder route if you aren't set on a kitten. That way you could have a full bengal with a reputable background for a nominal price. Ex breeders espcially are some of the most beautiful examples of the breed and are often offered to a good forever home simply for the cost of neutering / spaying!
Sorry i couldn't provide a way of guaranteeing you choose a bengal but i hope this information is helpful! Please keep us updated with your search and feel free to ask any more questions if you are unsure