An insulated panel comprises an external sheet 2, an internal lining sheet 3 and an insulating case 4 between the external and internal sheets. Indicating means are provided on an external face of a male projecting part 10 of the external sheet to indicate correct alignment of the panels at the joint. The indicating means may comprise a black line 26 printed on the outwardly extending leg 20 of the male projecting part 10. When the line 26 is aligned with an edge of an adjacent pane a visual indication is provided that the joint is correct.