There aren't any fancy gadgets and gizmos to help you. The Steel Structures Painting Council has issued some pictorial standards showing rusted surfaces, and what those surfaces should look like after different degrees of cleaning. Most of these are for abrasive blasting, but some deal with the more mundane methods employed by residential and commercial painters. Your inspection needs to be more than visual. Make sure you can't stick your finger through it.
Rust tends to attract other things, such as dirt, that affect adhesion.
Use a powerwasher if you can, or a wire brush, to get rid of the loose, flaking rust. Then clean and wipe down the surface. There are coatings on the market now, and more on the way, that can be applied to damp surfaces and still work to impede rust while forming a bond with the substrate. These kinds of coatings are so designed that when they dry, the moisture that's on the surface and in the coating evaporates. There also are ingredients in the coating that help prevent rusting while the water is evaporating.