...More on screw identification...

If it's a numbered screw (in inferial land most smaller than 1/4" are), it helps to know the number. I made a gauge for this purpose...

DIY Screw size gage

but if you don't do it often it may be easier to just work it out mathematically. For numbered screws the diameter is computed from...

D = 0.060 + 0.013 * N

where N is the screw number. Solving for N, we have...

N = (D - 0.060) / 0.013

so measure the diameter of your unknown, plug it into the above and solve for N.

If you don't already own thread pitch gauges (inferial and metric), remember that any screw of known pitch can be used as a pitch gauge.