What is a good way to tell what breeds are in a mix breed dog?

I just recently found out that you can have a blood sample of your dog taken so that they can find out the true breeds in your dog. I'm sure if you talk to your vet they might know where to get you this service.

Our shelter has alot of stray dogs also and we get more strays from another facility as well. We go with the closest breed we feel they are and are perfectly honoest with our adopters that we don't know the real breeding and history of the dog and base it on what we know and how the dog is with us.

As far as 'best guess', I usually decide which of the breeds common to the area share at least a couple characteristics (such as coat length/color, shape of head, ear and tail set/carriage, and general temperament).
Often you may find a stray that seems just like a rare breed, but in almost every situation it'll actually be a random mix.

The main times that really knowing the breed/mix makes a difference, is in terms of genetics and what can be expected in the future. A puppy may be little and sweet now, but they may end-up a 40 lb dog with Border Collie activity level, or a 150 lb dog with a nurturing Newfie personality... or anywhere in between.
Genetics also play a huge role in what health issues are likely to crop-up and knowing what is potentially there can really aid in speeding up the diagnosis.

I personally enjoy trying to ID mixes by analyzing the probability of different combinations of breeds meshing to create the dominant and recessive traits that are visible in that particular dog.

It is sometimes hard to tell, but you can determine by color, basic makeup, hair coat, and activity level. There is also a book that I saw at Barnes and Nobels that is all about figuring out what breeds are in a mutt. It breaks it down step by step piece by piece as to which breed is what in a mutt.

Look at the Size,Head-shape, Weight, and Coloring of the Dog. Its good to have a Book or computor handy so you can look up Dog breeds and see what it most resembles. Personalty's are another way to determine the Breed, Border collies have a strong drive and focus and very high energy level while Labs are hyper and easy going with a very affectionate temperament. I Volunteer at the Dearborn Animal Shelter in Dearborn Michigan and we run into the same problem with dogs at the shelter. Good luck. Heres a good website of different dog breeds: http://www.dogbreedinfo.com