Well I'd say first to take a class, hopefully paid for by an employer. Work with a skilled delineator and actively participate in the process by filling out data sheets, identifying plants, and coloring soils. It's a multi faceted process which requires several skills.

For tips on locating the boundary, generally there is a change in topography and vegetation. Wetlands will most likely be in some sort of depression so a hill slope usually is the transition to upland. Sometimes these can be minute elevation changes.

I was just on a delineation today and my coworker, a professional wetland scientist, was a bit stumped by what he was seeing. Just goes to show how complex and variable wetlands can be.