Well, PA is not "huge" and they are not a publisher, but this is preaching to the choir.
The validation you get when your work is accepted by someone, great or small, can be a life-changing experience. Maybe your work stinks, but someone's told you otherwise and sent you a check for it.
Dear PA Lurkers,
Which is the better validation for your hard work:
the one dollar check (with an option to frame costing only 20.00!) from PA,
or a 4,000.00 check from St. Martin's?
How do you know if your work is good?
You do what we all do, find a decent crit group, get feedback, write every day, and send out stuff until it sells.
It's like practicing the piano--do it often enough and you can tell when your playing is ready for Carnegie Hall--and so will Carnegie Hall!