I've read in some posts that the glue on the decals dries out with age. And I've actually experienced this on a bird decal I've had since the 80's. So here's the question ... how can you know decals from any source aren't going to be dried out old stock ?

Thanks,

I had NOS stripes installed on my T/A project, and it worked out. But it took extra work and care, and we didn't know for sure if they'd work out or not, until they tried to install them. My stripe kit was date coded in the 80s so it was over 25 years old when installed. I'd had them since 91 or 92 and kept them stored in a closet in my house.
I put some NOS bees in my 1/4 windows too, and they took a lot of work to get off the backing paper intact.
I think it's hit or miss with NOS decals and you take your chances. If using OE stuff is important to you over repro, that's what you have to do. Otherwise get fresh repro decals.

But what's "fresh" ????

If you buy new decals from a reputable source, then you assume they are fresh. And if they aren't, you return them for an exchange when after you find the problem.
With NOS ones, unless the decals are separating from the backing paper, you don't have a way to tell if they are any good until you try installing them.
The first shop that had my car said they wouldn't even attempt to install NOS stripe decals. He'd had a bad experience installing NOS stripes on a '69 Mach 1. Several months after installing them on that car, they started to craze or delaminate or something like that, so he decided he'd never install NOS stripes decals again.
My cars lower left fender got damaged unloading it from a trailer when liberating it from the second shop I had it in. The lower half of the fender had to be repainted, and I lost my 340 Six Pak decal on that side. A friend of mine who also has a T/A had an NOS decal he let me have, and it looked pretty ancient! My local body and paint wizard Dave shook his head when he looked at it, but he carefully installed it with me looking over his shoulder, and it looks great.
Guess I've been lucky, but to your question, I don't think there is a real good way to tell if a decal is "fresh" until you try to install.