The question is a bit broad. Provide more info to narrow the scope of the question, such as:
Do you have a standard to meet?
Are you testing compacted fill or do you want to know the in-place density of existing soil?
Is this for roadway or building work?
Will the testing be done at the surface or subsurface?
2) Perform a proctor ASTM D1556 on the soil being compacted in your local materials testing laboratory.
3) Use the proctor value and any of a number of methods of testing the compacted soil. The most common of these methods includes a nuclear density gauge, and the sand cone method.
Does you firm work with geotechs when designing your projects? I'm sure the geotech firm would be willing to give you or your office a little review session on soils basics. Usually geotechs provide recommendations to the structural engineers for use in designing the proposed foundations and performing required earthwork operations. It would be to everyone's benefit if each party better understood the other.
I second PEInc's comment. We can't give you enough information in this forum; you need to spend a (minimum of a) few hours with a local geotechnical engineer in order to get a handle on this topic.
Why not do a search on the various previous threads ? - there are a lot on compaction (e.g., Thread274-5909 ) Type in "Compacation" on "Key Word Search" and I think that you might find some useful information.