Well, there is a system in place already.
You simply place a inconspicuous LGBT ribbon decal on your store window or advertisement.
People in the know think, "Wow, they are LGBT friendly!"
The plus side is that your typical homophobe or transphobe doesn't know what it means so that the business doesn't lose their patronage.
Of course, if the business was truly supportive they would put up a sign saying,
"We support lesbian, gay, bisexual and transgender people.
If that's not Ok with you, take your business somewhere else."