How can I tell if what I'm writing is any good?

Dear Literary Ladies,
How can you guage, in the midst of writing, if your work is any good? It’s so hard to be objective, and see the forest from the trees. Should I compare my writing with that of other writers I admire?

Since we must and do write each in our own way, we may during actual writing get more lasting instruction not from another’s work, whatever its blessings, however better it is than ours, but from our own poor scratched-over pages. For these we can hold up to life. That is, we are born with a mind and heart to hold each page up to and to ask: Is it valid?

—Eudora Welty, from the essay “Words into Fiction,” 1965