An easy way to tell soil temperature is simply to stick a thermometer about 4" in the ground. You can take a measurement in the early morning and late afternoon to get a high and low for the day, then average them. K-State also has a good weather data library that shows soil temperatures at weather stations throughout the state.
When the soil temp is around 65 degrees, Petunias, Begonias, Lobularia (Alyssum) and Snapdragons can go in the ground. More sensitive crops like Vinca (Catharanthus), Celosia, Lantana, Melampodium, Zinnias, and Pentas need soil temperatures of 68-70 degrees. These are the flowers that thrive in the heat of the summer and need those high temperatures. Most everything else falls in between.
This is the year to have patience. Wait until soil temperatures are up for a couple of days before rushing out to plant. Our most heat tolerant plants tolerate the heat because they love and thrive in warm soils. Cool damp soils will comprise their root systems and then when the real heat of the summer does finally arrive, they will have few roots to support the plants growth.