Call around to different vets and ask them questions, aka screen them. Good questions to ask are:
How many cavies do you see and treat each month?
What is the extent of your medical training regarding cavies?
Who would you recommend for an after hours emergency?
Can you do an anesthetized dental exam? (Needs to say yes)
What types of antibiotics would you prescribe for an upper respiratory infection?
(Baytril, Bactrim, or TMZ, NOT any penicillin derivative, as these are toxic to small animals)
Does your clinic have hand-feeding formula such as Oxbow’s Critical Care and could you teach me to hand feed if my pig or rabbit was critically ill? (Needs to say yes)
Can Baytril be used on young pigs? ("We know the answer is that it is not recommended, there are safer alternatives for young pigs.")
What types of foods do you recommend feeding a guinea pig? ("I can't tell you how many vets told me to feed alfalfa on a daily basis!")
What would you prescribe for mites (ivermectin or selamectin, NOT Frontline or sulfur baths)
These questions were taken from
http://www.guinealynx.info/vet.html
and
The ACR&S: Recommended Vets