I am getting anxious to start seedlings.

most of you mentioned i shouldnt plant until the ground is above 70
deg, how does one test? bury a thermometer?

also, most of you said i should start by planting in my starter tray
4 weeks before planting in the ground. how do i predict that the
ground will be 70 degrees in four weeks?

also, would it hurt for the seedlings to be started early? how early?

I've heard of trying to pre-germinate before planting in starter
trays, can someone recommend a method?