Just from observation you can usually tell if the snake is in a good mood or not. Like any animal, they have better days than others. My boa Lexus (8ft) can be pissy in her cage UNTIL she knows I'm not feeding her. After that she is a doll.
Most boas have really sweet temperaments, and are pretty easy to handle. Make sure you support his/her whole body, and go slow. They don't like sudden movements, especially around their faces. I do think that's very important that you hold the snake, rather than let it hold onto you. The reason for this is because many snakes that are used to be handled start to rely on you to hold them. They become relaxed enough that they actually might fall. I would refrain from handling a snake unless absolutely necessary if they are about to shed, or in shed, or if its been less than 2 days from feeding.
If you are really worried about the snake, or if he/she is defensive in their cage, you can use a broomstick or something of the sort to gently pet them. I always do this with my girl since she has a very aggressive feeding response. Some snakes can be very defensive in their cages, but once they are out, they calm down and are docile.
If it's your first time handling the snake, I would sit down on a couch, or the floor. That way if you get startled and accidentally drop the animal, it'll just land on your lap. She/he may be a little tense feeling at first. They seem to know when an inexperienced person is handling them.
Most boas are real sweethearts, so I wouldn't be worried if the snake is known to have a good temperament.
If you have any more questions, please let me know!