when you sing you want to use your stomach and you want to open your soft pallet like when you want to yawn that is when you open your soft pallet.
If you want to know if you are a low singer or a high singer usually you are better at singing higher or lower so that is how you can tell.
when you learn the notes you sig than that is very important because that is how you will know when you have to sing high or low so notes when you sing are very important.