What counts as proof

Your Veteran Health ID Card (VHIC) or VA Veteran ID Card.
A Texas driver licence or state ID with "Veteran" printed on it.
A military ID or retiree ID card.
Your DD-214 discharge papers.
A VA disability rating letter.
Some restaurants also accept a VFW or American Legion card, or a photo of you in uniform.

The websites that check for you

Many companies will not give the discount until a website confirms you served. There are four. Setting up any of them is free:

ID.me — used by Lowe's, Verizon, Apple, Samsung, Six Flags, SeaWorld, Sam's Club, Discount Tire, Tractor Supply, Under Armour, Adidas, Columbia, Toyota, Hyundai, Ford, Vet Tix and many more. You set it up once and use it everywhere.
SheerID — used by The Home Depot, Target, Nike, Dell and Foot Locker. You usually verify right at checkout.
WeSalute (used to be Veterans Advantage) — free basic account, but the best offers (like Budget rental cars) need the paid plan. Read the price before you sign up.
GOVX ID — free, and taken by thousands of online shops. Verify once and you get a login you can use wherever you see the GOVX ID badge, and it opens GOVX.com itself as well. It covers veterans, military families, police, fire, EMTs, nurses and government workers.

No computer? This is the part to ask for help with

Setting up ID.me is the one step that stops most people. It needs an email address and a photo of your ID. A family member can sit with you and do it in about 15 minutes, or your county Veterans Service Officer can help. In Comal County call 830-221-1170. In Bexar County call 210-435-7838.

Find a free Veterans Service Officer Go to ID.me

Three things to remember

Always ask. Plenty of places give a discount and never advertise it. It costs nothing to ask at the register.
Franchises differ. With chains like Chick-fil-A, Jiffy Lube and most gyms, the local owner decides. Call your own store.
Some offers are active duty only. Those are marked on this list so you are not disappointed at the counter.