Here's how I learned what 90-100rpm feels like:

Go to Sheldon's gear calculator, enter in your numbers, and calculate for both MPH @ 90rpm & MPH @ 100rpm. Either print the sheet out or remember the numbers. Then, when you're riding, take note of how fast you're going, and then switch to the gear that allows you to maintain that speed at 90-100rpm. After a little while, you'll start riding at that cadence naturally (if you aren't already).