When I'm eavesdropping on other peoples conversations at ultrasound courses, I often hear people talk about compressing the vessel you want to cannulate to make sure its a vein and not an artery (if you happen to be placing a peripheral IV).  Here's the thing: Peripheral arteries and veins can both be compressible.  

I suggest looking for pulsitility instead of compressibility when trying to differentiate between the two. Check out the podcast for more deets.