Though you don't typically need it, add_argument returns an instance of the Action subclass used to implement the argument. Among other things, these objects have a dest attribute containing the string you want.

For example,

import argparse

parser = argparse.ArgumentParser()
a1 = parser.add_argument('arg1')
a2 = parser.add_argument('arg2')
a3 = parser.add_argument('arg3')
a4 = parser.add_argument('arg4')
     
destinations = [x.dest for x in [a1, a2, a3, a4]])

parser._actions contains a list of all arguments defined for the parser, including things like --help, so that may also be of use.

Thanks to the information provided by @hpaulj in the comment to the OP, I managed to get this working.

Basically, you need your main parser and a parent parser. You will then set the parents attribute of your subparsers to be the parent parser. Based on the example you gave, the following should be a working example:

import argparse

# Create parsers
parser = argparse.ArgumentParser() 
parent_parser = argparse.ArgumentParser()

# Add arguments to parent parser
parent_parser.add_argument("--foo", action='store_true')

# Create subparser
subparsers = parser.add_subparsers(dest='cmd')

# Add to the subparser
bar = subparsers.add_parser("bar", parents=[parent_parser], add_help=False)
bar.add_argument("--test", action='store_true')
baz = subparsers.add_parser("baz", parents=[parent_parser], add_help=False)
baz.add_argument("--baz-test", action="store_true")

# should be able to have '--foo' before OR after 'bar'
print(parser.parse_args(['bar', '--test']))
print(parser.parse_args(["baz", "--baz-test"]))

This outputs the following:

Namespace(cmd='bar', foo=False, test=True)
Namespace(baz_test=True, cmd='baz', foo=False)

You should then be able to do things like this:

args = parser.parse_args()
if args.cmd == "bar":
    print("bar was specified")
elif args.cmd == "baz":
    print("baz was specified")

It might not be the perfect solution, but it should work.

(Tested using Python 3.5.2)