I would start by taking a look underneath for any wetness, because oil leaks given enough time drip 'down'. Look in the front and look in the rear, in case oil is coming out of the transmission. Whatever oil you see (if any), then track 'up' to the source. take any covers off the top of the engine and look (if necessary). Most common leak points are where oil lines join, intake and valve cover mating surfaces and any items that rotate, like the crankshaft, driveshaft outputs, rear diff. etc. Could even be something simple like a bad seal on the oil drain plug, or it wasn't tightened up enough last time oil was changed.
I know I'm not giving you specific locations, but there are so many potential leak points, you just gotta put your eyes on it. It's either dripping onto the exhaust down below, or dripping on the exhaust manifolds up top, and causing smoke. It only takes a small drop or two.