I can understand why you picked this – you are looking at the father’s side and saying “I don’t know if they are a carrier or not”. Well, in this case, that actually doesn’t matter to the answer (big hint!).
However, a more important point, generally for these types of scenarios the question would likely say a “rare” disorder. If it does, you can assume that is there is no family history, then they probably aren’t carriers.
Rare (in the UK) is defined as an incidence rate of fewer than 1 in 2000 people. This means, for recessive disorders, an approximate maximum carrier frequency of 1 in 500 as 1/4 of het/het offspring will be homozygous.