Found an old glass bottle buried in my yard while digging a footer - who do I even ask about this?

A blob top with greenish glass is usually somewhere in the 1880s to about 1920, so you're not far off with the 'somebody's trash' guess, though old trash in a bottle guy's eyes is still treasure. The seams are exactly where you want to look. Run your finger up the side of the bottle. If the seam lines stop at the base of the neck, that's a hand-finished bottle and it leans older. If the seam runs all the way over the top of the lip and across it, that's machine made, which puts it roughly 1910 or later. Blob tops specifically mostly died out by the 1910s when crown caps took over, so yours probably predates that. Flip it over and check the base too. A rough scar or blob of glass right in the center means it was snapped off a blowpipe, which is the hand-blown era. A smooth base with a mold seam means machine. Little bubbles and wavy glass also point older, though that's less reliable.

As for digging more, don't just go at it blind. What you want to find is the actual deposit, which is usually a pit of dark loamy soil, ash, oyster shell, broken dishes, and a whole cluster of bottles in one spot. That's where the good stuff concentrates. If you're already trenching for the footer and you notice the soil change color or get stinky and ashy, slow down and poke around there. A screen with half inch mesh helps a lot for the small stuff.

One thing though. If you turn up anything that looks like bones or a lined brick structure, stop and don't just pull it out. Old privies and wells are where the best bottles come from, but you want to be sure what you're dealing with before you go further. Otherwise, keep an eye on the spoil pile as you dig the footer. Half the bottles I've found over the years were sitting in the dirt I'd already thrown out.

Those partial letters on the side are actually your best friend here. I dug up a couple embossed bottles on a job a few years back and typed the readable bits into a bottle collector forum, got an exact date and use within a day. Somebody out there recognizes that lettering style instantly. Also, before you go hunting for more, check wether your town or county has a records office with old sanborn maps or aerial photos. Ours had the farm road on them and it explained exactly why the bottles kept showing up along my trench line.

We dug up a bunch of old broken plates and a doll's head when my dad put a garden in years back, and my mom just kept them in a shoebox forever. Never thought twice about it until reading this thread. Now I'm wondering if my whole neighborhood was like that under the grass. Random question, does the county or the state ever get a say in stuff like this, or is it basically finders keepers once it's on your own property? I genuinely have no idea how that works.

somethig I learned the hard way on a garage slab job a few years back - write down the depth and where on the lot you found it before you forget. Every bottle collector I've ever shown something to asked me how deep it was first, because that tells them whether it got buried with the trash or just migrated down through the years. I had a guy date a stoneware jug I pulled from 3 feet down partly off the depth alone. A phone pic of it sitting in the trench wall with a tape measure in the shot is worth more than the bottle itself.

Gotta push back on the bubbles thing though. Wavy glass with bubbles doesn't tell you much on its own, I've got a machine made mason jar from probably the 50s that's got bubbles all thru the base, every glass plant had sloppy days. If you want a better check, set the bottle on a sunny windowsill for a week or two. If it starts turning faint purple, that's manganese in the mix and that stuff pretty much disappeared by WWI. Clear staying clear leans later. Also on the dig-more question, I'd pass on the screening. Your footer isn't going to wait while you sift spoil piles, and poking sideways into soft old fill next to an open trench is a good way to undercut the hole you just dug.