The first time machine builder...
Had better test it in a large, open area!
The last theory I read said that if you build a time machine, you can't travel any further back than the first time it's first turned on. So... the best test that a time machine works is that someone from the future pops out of it to witness the first time machine being activated.
Of course, thousands, possibly millions of time travelers will want to witness such an historic event, so there will be a flood of people popping out of it to witness it's creation.
I'd recommend plenty of restroom facilities and a good caterer!
On another note:
Say Bob has built a time machine, but keeps it locked for security reasons.
Jim wants to use it and asks Bob for the key.
Bob: "Ok, but I have an experiment ready to go in two minutes."
Jim: "No problem, I'll come back before I leave!"
So, just as Bob reaches for his pocket to get the key, Jim pops out of the machine, back from his trip, and tosses the key to The Jim that hasn't left yet, who then enters the machine and leaves. (You still with me?)
Now here's the question: If Bob reaches into his pocket, is the key still there?