Tools and methods are used to determine the oil, water, and solids volume fractions in a drilling fluid at the rig site. The volume fractions can be determined in-line with returned drilling fluid by using an NMR magnet and a flow line or sample chamber that receives a fluid sample and loads it into the NMR magnet. Using an RF probe, spectrometer, and computing device, data processing and interpretation of NMR data from the spectrometer is performed, while also raising a flag when iron contamination exceeds a predefined threshold.