Written references are ok, but they can’t show you the kind of workmanship that will be performed, nor can they guarantee that your installer is the same as the installer in a written reference. If at all possible talk with a previous customer. Windows must be installed squarely to ensure proper performance, an easy way to check this is to open 1 sash (on either a horizontal or vertical sliding window) then slide it slowly toward the main frame until there is a slight gap between the two. If this gap is uniform it indicates the window has been installed squarely, if the gap is wide at one side and small at the other, the window is improperly installed. Look at the mouldings that were installed with the windows, are the miters tight? On the exterior, look at any aluminum capping that has been done, are the joints tight and fastened with screws? Is the caulking neat?. Ask if this person’s home was protected using drop sheets. Was the property left clean? Did the installer demonstrate how to operate your new windows?