The simplest way is to read the documentation, and you will find that every graphics function returns the handles of the graphics objects that it creates. The plotting functions also accept an axes handle, so you can specify exactly which axes to plot in:
fgh = figure(...)
axh = axes(fgh,...)
lnh = plot(axh,...)
Never rely on gca, gcf or anything similar: they are unreliable and will make your code buggy because their behavior depends on what the user clicked on, or what code has just run... basically using gcf and gca is how beginners write buggy, unreliable code. They should be used for playing in the command window, not for any serious code.
