If your LHBS made the recipe they can definitely tell you what the FG should be.
I take a sample of my brew with a sanitized turkey baster put it into a Hydrometer Jar (tall skinny tube of plastic w/ a bottom). You can do it in any tall cup though, you just need to take a larger sample.
Drop in the Hydrometer, spin it a couple times to clear any bubbles from the bottom and see where the liquid level is on the meter.
If the bottom tip of the hydrometer is touching the bottom of the glass you need more fluid till it is fully suspended in the sample.
Don't forget to adjust the gravity based on the temperature of the liquid, a table of adjustments should have come with the Hydrometer.
If you post the recipe I can run it through BeerSmith and get an estimate for the FG.