A method for determining an asphaltene onset condition of a crude oil is provided. The method includes receiving a crude oil within a downhole tool inside a well and taking a first measurement of an optical property of the received crude oil. The method also includes lowering the pressure or temperature of the crude oil after taking the first measurement of the optical property to cause aggregation of asphaltenes in the crude oil, and then separating aggregated asphaltenes from the crude oil. Further, the method includes taking a second measurement of the optical property of the crude oil within the downhole tool after separating aggregated asphaltenes from the crude oil and determining an asphaltene onset condition of the crude oil through comparison of the first and second measurements of the optical property. Additional methods, systems, and devices are also disclosed.