It depends on the mutation of the parents. Only in certain mutations is this possible, if the parents are also a certain mutation (Only certain scenarios) . (Sex linked mutations like cinnamons, pearls, lutino) . Here is a website to help you understand mutations.
Cockatiel Genetics
To give one example where it is possible:
Mom bird does not visually show a sex linked mutation. (She is NOT a cinnamons, pearls, lutino)
Dad bird has visually shown a sex linked mutation. (He is a cinnamon/pearl/lutino)
Then we know, All babies WITH a sex linked mutation must be female. All babies without the visual mutation must be male. This is because in sex linked mutations, both parents must have the mutation or dad must at least be split to have male babies that have sex linked mutations. Since in this scenario only DAD has the mutation, then we can confirm the babies must be female with the mutation and all the males are the ones without the mutation. Females ALWAYS show this mutation if at least one parent has the gene.
Yes I think it would be better to get a health gaurentee. (If one gets sick would she attempt to refund/pay vet bill or give you another bird in case it dies?)