Question
Can you give me the contact info for the closest, cheapest Soil Test.  I live in Suffolk County on Long Island in New York. I put down new soil and seed last fall in area between sidewalk and street.  It came up with alot of weeds.  Used weed and feed 1 week ago and parts of lawn are yellowing.  Plan on running alot of water on yellow spots.  Thinking to use spot weed killer on edges where weeds are concentrated.  Read an article you wrote about getting soil tested.  What to do??? I am fed up with this lawn.  Lost it last year from spraying lawn with weed killer that attached to hose.  Did it twice between spring and summer.  Then had bay water flood area and lost lawn between street and sidewalk.

Answer
Hi Anna, Many garden centers will test your pH for free and they usually sell an inexpensive soil test kit that you can check your pH, nitrogen, phosphorous and potassium levels.  They will not tell you however if you have a fungal or other soil issue.  You would need to talk to a local analytical lab or possibly your local state university's agricultural extension office.
Be careful flooding the area, if it is fungal or if it is suffering from herbicide damage, excessive water may make the problem worse.  Water deeply but infrequently.
Several things you said concern me. "New soil" should not be different than existing soil.  Layered soil of different types, often causes shallow root systems, areas that are too wet or too dry and subject to winter kill and water stress.  
If it were mine, I'd start again.  Till the new soil into the old soil to get a uniform consistency.  If it's mostly weeds, I'd spray the whole area with glyphosate (Round-up, Kill-zall) a few days before tilling.  It does nothing to the soil so you can re-seed in a couple of days.  After tilling, check your pH.  It should be in the 6.5 to 7 range.  If it's not, ask your local garden center for the products to adjust it.
I'm guessing you are sowing fescue.  Fall is a better time to plant, but if your summers are mild, you may be able to plant a stand now.  Again, ask your local expert about the timing.
Ignore the weeds for the first full year.  We can always get them out, but your young grass does not need the added stress of herbicides until it is well established.  Come back in a year and we will address weeds.
It is very important to water properly...deeply and infrequently.  I cannot say that enough.
If you don't want to start over, fertilize only twice a year.  Once in the spring and again in the fall. Back off the weed killers until we get the grass healthier. Your local garden center should have either a liquid or dry source of humates.  Buy it and apply it every 6 weeks for a total of 4 applications. If you haven't fertilized, do so at a rate of 1 lb of nitrogen per 2000 sq. ft. Use a slow release or organic nitrogen, no "agricultural grade" fertilizers.  Let me know if the color and vigor improves. Jim