When I am going through petfinder dogs, how do i tell the difference btw a rat and a mislabeled toy fox terrier?  On google image search, they look pretty close

Big difference, especially in the face.

After 8 years in Rat Terrier rescue,  I still cannot easily tell the difference between a Rat Terrier and a TOY Fox Terrier.  The Toy Fox is much different in appearance from the Smooth Fox Terrier but quite like a small Rat Terrier.  Size is a big factor although there will be crossover.  TFT should be 8 lbs or less.  It is not unusual to find them, registered PB that are 10-12 lbs.  Bigger than that they pretty much will be passed on by TFT rescue.  They tend to have a little more of the rounded head,  bug eyes and distinct stop than a good RT but this is also not absolute. 

I have one dog I adopted 8 years ago as a Rat Terrier but I believe her to be a TFT.  She has very short, silky hair, the rounder head and she has a MUCH feistier disposition than most of the Rat Terriers I have had.  She is only 10 lbs but she will light into any fight that starts and has to be drug up by whatever part I can reach because she is like a Pit Bull when she fights.

I hope this helps a bit.  If you see a specific dog you want to identify at any time feel free to contact a Ratbone member.  Several are quite good at telling the difference. 

Dot Lewis, mom to many

Dot is right, TFTs and RTs can be AMAZINGLY similar in looks. However, usually (and of course there are exceptions) TFTs are fiestier, like half way between an RT and a JRT for temperment. Zip (my avatar) is supposedly a purebred (came with papers even when he entered rescue) TFT but he is very tall and 15lbs. Peanut (purebred RAT in rescue) has the same build and lines but RT coloring. Roxy TFT looks more CHI in the head which is common in many TFTs.

TFTs seldom have the wedge head that RTs can get, they are rounder or finer lined. But there are so many exceptions in both breeds (plus TFT lines are in the RT breed) that it can be tough to know for sure.

If you look here:

http://www.atftc.com/success.html

you will see how similar many look.

Also here: http://www.atftc.com/page1.html

For what it's worth... I had a Toy Fox Terrier for 9 years (she is still around, but stayed at my mom's house when I moved out).  I LOVE her, she is very good natured, about 8 pounds.  She is the reason I got my rat terriers.  I was looking for TFT's in resuce when I came across the 2 ratties that I ended up adopting.  I looked up Rat terriers online prior to the rescue and it seems like they are pretty similar to TFTs, the main differences seemed to only be important for showing a dog.   RT descriptions seemed to be more good natured/smart dogs, but my TFT was (and still is) patient with children, other dogs, adults, she is smart, too, learned tricks and comes when she is called, doesn't run away, loves to go on walks... my RT's are still only 8 wks so it is hard to compare them to the TFT yet.  I honestly think you can't go wrong with either pet!

Yes, I know and I agree the TFT are hard to tell from toy ratties.  I didn't read the poster's post well enough and just saw "fox terrier" and jumped on it - my bad!