There are a few ways to do this.
Look out for stains and discolouration. Ceilings, walls, around windows and doors. They may be a variety of colours and shades to take note of anything that looks out of the ordinary. It could even be an idea to take some photos with each inspection so you can refer back to what it looked like previously. Other things to look out for with your visual inspection are rust, bubbles, peeling paint, warped materials including flooring, furniture and cabinetry.
Press on walls, floors and the ceiling. If a surface that is supposed to be hard feels soft or spongy or even give way you can put the red flag up. Depending on your smelling senses you might pickup on a damp, musty smell that might be coming from an area that you can’t see.
Take a closer look at seams, seals and where there is any change in material. Run your fingers along the seals to make sure they are in good condition. Have a closer look around windows and doors, inspect plumbing fittings, pipes and drains. During a heavy downpour, if you can easily access your van, take a look inside and do your own water test.