You could ask at your vets for any recommendations. Our vet knows the cattery we use very well.
When you visit some catteries for a look around, find out how they deal with a suddenly ill cat while you are away, and if they will give any medications? You will probably need to sign a permission form to agree to any emergency vet treatment.
Will they groom the cats if long haired?
Will they give the diet your cats like, or do they all get the same?
Do they check that your cats are vaccinated, and ask to see the record card every year to make sure their boosters are up to date?
Will your cats share an enclosure, and is there a discount for this? It can be expensive to house two cats separately.
Do you like the owner, do they have a real passion for cats?
Do you like the facilities on offer?
From my own experience, don't leave it too late to book the cattery, the popular ones are very busy and often full in the summer. Take the cats own beds and toys to make them feel at home.