Hello!
If given a graph, you look for the point furthest to the left, and the point furthest to the right. For range you do up and down.
If you are just given a function, you can think of a few things. If you are given a radical, it must not contain a negative number. Therefore, the domain and range for that function would not contain any negative numbers; they would be greater than or equal to 0.
Also, if you have a function that involves division, you cannot divide by 0 as it is undefined. Therefore, the domain could be any real number except for 0, or a certain number that would keep any other numbers in the dividend from becoming 0.
I hope this helps!