I dont think you test for it unless your dog is showing signs of it, limping.
leg perthes is an adolecent disease, which means if your dog doesnt develope it within the first year, chances are your dog wont develope it at all.
The best thing to do is to really familiarize yourself with signs and symptoms of diseases. here is a good link to canine diseases.
http://www.handicappedpets.com/genetic/diseases.htm
Hope this helps!