Soil compaction diagnosis is not as straightforward as it seems, especially because we lack good tools to do a quick test.
The standard tool is the penetrometer, or soil compaction tester. It has a pressure gauge, a solid rod and a standard tip (usually of 0.5 inch diameter at the top). It has to be pressed into the soil at approximately 1 inch per second when soil is at field capacity (a day or two after soil profile is completely soaked).
Measuring at field capacity is necessary because penetration resistance will increase when soil dries out. Root growth, however, will resume once the soils wets back up. The gauge tells you if you reach 'root limiting' compaction.
The tool was developed in completely pulverized soil packed to different densities. Seedlings were grown in these soils and it was found that roots did not penetrate soil if the penetration resistance exceeded 300 psi, and were starting to be inhibited when penetration resistance exceeded 200 psi.
In tillage literature you will find that if penetration resistance is less than 100 psi seed-to-soil contact is likely to be poor due to excessive air pockets (so soil that has been plowed has to be packed to 100 psi to get good seed-to-soil contact, that is why we use rollers and packers).
In conclusion, penetration resistance should be between 100 and 200 psi. More information on the use of the penetrometer is found in Agronomy Facts 63 available from Penn State Extension.
While the penetrometer can give you a first assessment, it is not advisable to base your conclusion solely on it. This is especially the case in soil that has not been tilled. In long-term no-till, sod or forest soil the penetration resistance may exceed 300 psi and yet there may not be a compaction problem.
The reason is that in these soils firm aggregates are surrounded by a network of pores that allow root growth, aeration, and water percolation to take place. So in addition to using the penetrometer, you need a shovel and dig to a depth of 12-18 inches to assess soil structure.
You need to determine if the soil is massive or crumb. If the clods fall apart easily or are held together by dense root networks, that is a first sign compaction may not be problematic. Look for severe platy structure in the surface soil. Then you need to search for evidence of root growth restrictions. Look where there is living vegetation in the field (for example, weeds or cover crop), and determine if root growth is limited - in compacted soil roots typically follow cracks without being able to grow into the massive clods, or the roots crowd in the horizontal voids between massive plates.
Do you see evidence of shiny surfaces created by tillage tools working in wet soil? If there is a hard pan, roots may make an abrupt turn or many fine roots crowd above this layer. If roots grow downward without a problem, compaction is not likely to be severe. Also look for organic matter content - does the soil show evidence of organic matter accumulations, which lead to soil becoming better aggregated and crumb? Finally, look for biological activity, such as that of earthworms.
At the surface of the soil, you can look for middens (click here to see the Earthworm Factsheet) - underneath each burrow there is a 4-5 foot deep open nightcrawler channel so if you have many middens, there is not likely to be a compaction issue. Other worms dwell in the surface of the soil and fill their channels with casts as they go.
If you have many earthworms that is more evidence that compaction is not problematic. Good times to look for earthworm activity is in spring and fall when the soil is moist.