It is known by scholars and others in the field of Art History that Caravaggio executed his compositions without the use of preparatory drawings. Instead he made incisions on the wet priming layer to mark specific points in the painting, along with loose sketching called "abozo." Many such incisions are visible in this work, around the ear of St. Andrew and around Christ's head, shoulder line, eyebrows and lower sleeve.* X-Rays also revealed that Caravaggio made changes to the painting's compositions as the work progressed. Such changes are not normally found in copies.
Other characteristics of that lead experts to believe this was an original Caravaggio was the execution of priming and it's use in the overall painting. In this painting Caravaggio used a warm brown ochre as primer which is left exposed in some areas of the painting as a middle tone. It is also the use of Caravaggio's high constrasts of light and dark which made it an ovious orginal. Before cleaning the layers of dirt had neutralized these tones to soft monochromatic shades of brown, but as soon as the varnish and grime were removed the paintings true colors revealed it's identity as a painting by Caravaggio.
"There is no record of Caravaggio painting such a subject, so the Royal Collection work can only be dated stylistically. The brushwork is deceptively simple, giving an extremely convincing impression of form from a distance. The Calling of Saints Peter and Andrew seems to belong in the period of 1602-6, before Caravaggio fled Rome, a period when the sensuous surface detail of his earlier work gradually gave way to a spare, dark and expressive manner."*
*The Royal Collection, London, UK