Use an old-fashioned notebook. Use a smartphone or even an iPad. Whatever you like best, head out to the field, especially to the test plot on your farm, and note those differences between hybrids that you can determine now. Don't wait until harvest and check the yield monitor. It doesn't tell the whole story. A hybrid that does well this year may not do well in a year that exposes its weaknesses. You can see those strengths and weaknesses exposed in many cases if you check now.
For example, one knock on some hybrids last year, especially when they were placed under stress, was that they dropped ears. Whole ears lost on the ground cause yield losses to mount up quickly. In a good year like this one it might not happen, but with stress the weakness shows up. Just think back to '12.
What does show up this year are hybrids that have longer attachments from the ear to the stalk, and hybrids that allow ears to stick out at a wide angle from the stalk, even during pollination. Add a wind storm, a little ear rot, maybe some dry weather before harvest that stresses that connection to the ear, and the ear may be more likely to drop. Other hybrids hold ears close to the stalk, or close to the vest, so to speak.
You can see that now, and you can make notes now about which hybrids are better at holding on to ears. You can also note ear placement. Some hybrids place ears very high, especially when growing conditions are good, as they are this year. Is it too high? Will lodging be a problem?
Some hybrids have large tassels with many branches, while others have smaller tassels with fewer branches. Supposedly those are more efficient. You don't have to read about this trait and take someone's word for it. You can determine it yourself if you head to the field. Take a notepad or computer with you – your choice.