1. A method, comprising: conveying a wellbore tool in a wellbore extending into an earth formation, wherein: the wellbore tool comprises a fluid sampling device; the fluid sampling device comprises: an inner probe configured to withdraw a first fluid sample from the formation; and an outer probe surrounding the inner probe and configured to withdraw a second fluid sample from the formation; urging the fluid sampling device into contact with a wall of the wellbore; withdrawing the first and the second fluid samples from the formation while: maintaining the inner probe at a first pressure; and maintaining the outer probe at a second pressure, wherein the first pressure is higher than the second pressure; determining a first contamination amount of the first fluid sample using an optical fluid analysis (OFA) sensor; determining a second contamination amount of the second fluid sample using the OFA sensor; determining in real-time a difference between the first and second contamination amounts; confirming correct operation of the fluid sampling device by determining in real-time a maximum of the difference between the first and second contamination amounts; extrapolating future values for the difference between the first and second contamination amounts by: normalizing the first and second contamination amounts using initial values of the first and second contamination amounts determined upon commencement of withdrawing the first and second fluid samples; and fitting to a power-law model the determined difference between the first and second contamination amounts; determining from the extrapolated future values a time at which the difference between the first and second contamination amounts will reach a minimum; continuing withdrawing of the first and second fluid samples until reaching the determined time; and then initiating collection of a portion of the first fluid sample.