How do dealers know if a car is reliable? Do they test them out a lot?

with 140,000 miles it's definitely been driven around. It really depends if if was taken care of its entire life, but that is impossible to determine. Some people abuse their cars causing premature engine failure. Check the bluebook value to make sure you aren't getting ripped off. Check under the car and in the engine compartment for leaking fluids. 

140K on a PROPERLY maintained CVT is possible but VERY few people service their transmissions.

People say x car is reliable and y isn’t. What they are really saying is 95% of car x don’t break down , but only 80% of y don’t.

However, it doesn’t take a genius to realise that someone will have one of the 5% unreliable x, and many will have one of the 80%  reliable y.

So you can just be unlucky and have an unreliable example of the worlds most reliable car, or be lucky and have a reliable example of an unreliable car.

It’s just statistics.

They do not test anything.  They put a new battery,  clean the engine and interiors and put a price sticker on it.  You are about to be the first to get fooled.

The 3.5 liter motor has timing chain problems. If the oil looks dirty or is low think twice about buying it.

NEVER TRUST A SELLER. Do your due diligence and have any used car you intend to buy thoroughly checked out by a competent and unaffiliated shop. It costs money, but it's worth it.

Do you know how to tell if a used car salesman is lying? His lips are moving. 

They don't know anything - but will tell you anything they think you want to hear. 

If you are interested - pay to have a pre-purchase inspection - a good mechanic will give you a list of current issues -- if they do it right they will test compression and bleed down and coolant pressure test. If there are no documents with the car that say the time belt has been replaced, or other regular maintenance  has been done don't buy it,