Polymeric adhesives play a critical role in engineering applications, whether it is to bond components together or to serve as matrix for composite materials. Likewise, adhesives play a critical role in natural materials where adhesion is needed (e.g. mussel byssus) or to simply preserve the integrity of natural composite materials by holding fibers together (e.g. extra-collagenous proteins in bone). In this work we use a newly developed technique to measure the cohesive law and toughness of adhesives which is similar to a standard double cantilever beam configuration, but in which the beams are replaced by two rigid blocks. We originally developed this method for extracting the cohesive law of soft and weak biological adhesives, and we here show that it can be modified to include high strength of engineering adhesives. Using this method, the cohesive law of the adhesive is directly computed from the load-deflection curve of the experiment, without making initial assumption on its shape. The cohesive law reveals the strength and extensibility of the adhesives, which is richer in information than the toughness (which is the area under the cohesive law). We also define a non-dimensional parameter which can be used to quantitatively investigate whether the assumption of rigid substrates is valid. For values of the parameter close to unity, the RDCB rigidity assumption is valid and the method directly yields the cohesive law of the adhesive. The engineering and natural adhesives we tested showed a wide range of strength, toughness and extensibility, and revealed new pathways which can be exploited in the design and fabrication of biomimetic materials.