Methods and apparatus are described for improving fit checking at the bonding interface of a first component, such as a fan blade (1), and a second component, such as an edge guard (2) for the fan blade (1), to be bonded by adhesive. A bondline thickness profile over the area of the bonding interface is determined by interposing a compressible, flexible sensor layer between the components before they are bonded. The sensor layer contains an array of piezoelectric elements which indicate local bondline thickness by signalling pressure due to compression of the layer. The bondline thickness profile can be processed by a programmed control processor to produce an adhesive application schedule prescribing the shapes of pieces of adhesive film which, when applied over the bonding interface, will build up an adhesive layer corresponding in thickness to the bondline thickness. The sensor layer can be prepared as a kit of shaped panels or as a contoured preform (55) matching the form of the bonding interface.