How to know if the mechanic is telling me the truth?

My 2002 ford f150's breaks went out. Mechanic says I have to replace both rear rotors, pads and the break cable assembly. According to him the break cables can't be purchases at an auto store only dealership so just for that is 600.00. Total he wants to charge is 1300.00. Thats as much as my mortgage pmt. Can anyone tell me if that is resonable.

Nonsense. The Brake cable is about $10 at any auto parts store. Get a second estimate and find another mechanic.

If you don't trust your mechanic, then there are others out there. Go get a second opinion.

Oh… And the word is spelled BRAKES. Not "breaks"

Find a new mechanic. This one is buying a boat and needs your money for the payments.
Its brakes.

By getting a knowledgeable friend to have a look first!

Take it somewhere else and get a second opinion - dealers are more expensive per hour but fight hard to maintain a good reputation in the community - not saying they are all honest (up to the person assigned to the car) but more likely to do only what is required.

FYI Front Rotors (Motorcraft) <100 each, Pad set 60, so the actual parts cost of the rotors and pads (this is retail now the mechanic is making money on this price) is $260 even if the $600 part was needed he would still be making 650 for a job that probably will take an 1.5 - 2 hours that is a pretty sizable hourly rate.

Get price from another shop.
$600 for the brake cable is not reasonable. Brake cable from another truck/car will do just fine, but that is just me.
The brake cable is used on the rear, drum brakes. For that much cost, you can replace them with the disk brakes. Disk brakes are better for sure. Any honest mechanic would have recommended you that.
Rear only >> $100 for the rotors and $50 for good set of brake pads. I can find them cheaper on the internet.

Just have it towed to the dealer and they will make sure to get it fixed.