A very simple way is to search the internet and find listings of elements according to melting point, as there are also listings of them by boiling point as well. Some of the elements are actually not well enough known to have a listed melting point. If you don't have access to that information, you can get a relatively close idea of an element's melting or boiling point by knowing which group of elements it is in as many of them have similar transition points on a temperature scale.
you can also estimate them. Just look at one element and look at the boiling/melting point of all the 4 other elements that surround the element you want to know the boiling/meling point. To the Left, Right, Top, and bottom of it. You take those boiling/melting points and average them out. You'll get close to the actual number.
its easier to look on the internet for them...but you can use this method if you already know the other mlting/boiling points.