How could I tell what grade of Luxating patella my chihuahua has?

He usually walks just fine on his back right leg, then all of a sudden he would start holding it up, showing no signs of pain, and continue walking/running. He only does this for a short period of time.

Only a vet can tell you the grade of luxation. We could guess all day, but this really needs veterinary attention. Here's a good link on it: http://www.marvistavet.com/html/medial_luxating_patella.htmlWhat you are describing certainly sounds like luxating patella. Regardless of the grade eventually assigned by your vet, you want to start your dog on all the chondroprotectives right away, to delay arthritis in the stifle joints (knee joints). Because the joints move abnormally, they WILL eventually become arthritic, but you can do a lot to delay arthritis. -Keep him at his ideal weight or even slightly below, to reduce stress on the joints. -Put him on a good glucosamine/MSM supplement. Talk to your vet about the dosage to aim for, but it's usually cheaper to buy these supplements somewhere other than your veterinary office. I am a big fan of Glyco-Flex III, but there are others out there. -Put him on daily fish oil supplement. This provides numerous health benefits, but acts as a mild anti-inflammatory in relation to joint problems.-Make sure he gets regular, mild exercise, like leash walks. You will probably need to limit jumping and extended hard running going forward. -Don't assume that surgery is necessary if your dog is Grade 3 or higher. Mine is Grade 3 in one knee and grade 1 in the other. She did not need surgery. We began all the supplements immediately and restricted her activity appropriately (she has always been at a good weight) and she is now 8 and still not on a full time NSAID, although she does have one for occasional use. She will eventually need to be on a full-time NSAID, but the vet doesn't think she needs it yet. So hang in there--there's a lot you can do to help your dog!ETA: Avoid aspirin as a pain reliever in your dog. There are a lot of excellent reasons not to use aspirin in dogs long-term, but the one most important to you is this: aspirin is an indiscriminate COX inhibitor and will destroy cartilage at doses high enough to control pain. Your dog is prone to arthritis if he has a luxating patella, so your mission in life is now to PRESERVE cartilage. Aspirin is off the table for you.

If you've forgotten, give the orthopedic vet, that your vet referred you to, a phone call.

X-rays need to be sent to OFA or the like to be graded accurately. Your vet can probably give you a good idea of how bad it is