Here are a few things you should look for when at an open inspection

I have been to many open inspections, often as a favor to a family friend or relatives who are after a little help with one of life's big decisions. 
There are some things you can look for yourself when looking around a house when you have short limited time at an open inspection without the expensive inspection equipment that we carry. You may need a decent pocket torch and a decent zoom camera, for looking at the gable lead flashings from the road kerb prior to the inspection.

• Roof sheeting issues can often be seen from the road or on Google Earth satellite images, you can research the property prior to going. Having a look at the property from above can show rust or patching on the roof sheeting. It is more difficult to see issues that relate to tile roofing, however. Tile roofing leak issues are often due to lead flashings that have cracked due to age or rusting issues on flashings around skylights and evaporative air conditioners. Cracked or loose hip cap tiles or pointing can also contribute to leaking, as can other tile gaps caused by sagging hip rafters or a poorly supported ridge. Unfortunately, some things require an on roof inspection to find issues that can cause water ingress damage.

• Maintenance on painted timberwork, such as windows, can be seen with your eyes; however, touching with your hands will show you if some areas have just been patched over and painted to hide wood rot issues. Most wood rot is found on timber windows and the fascia beams of the house. Look at the mitre corners under the guttering and at any gutter joint, as this is where the gutter water will leak from and cause rot. Often the scotia timber trim under the guttering will have wood rot, as will the fascia beam behind the scotia where they touch. Painters do not paint where you can't see, so the back edge of the Scotia timber trim and the top of the fascia beam are nearly always unpainted and rot very easily. Modern houses use a H3-treated timber called LOSP for fascias and gable end treatments. This treated timber requires sealing with a tanalized finish on all cut surfaces, followed by painting to seal in the tanalized treatment as it is solvent-based and will evaporate away over time if not encapsulated in paint
  

• Ceiling stainings, popped nail heads and dropping ceilings can generally be seen with a high power torch. I would recommend you carry one in your pocket and casually check out areas as you go. common areas of concern are garage ceilings, as in most cases no insulation is installed in a garage ceiling, as it is not required to as a non habitable room. During a building and pest inspection I use a thermal camera to look for moisture issues, but water staining is often visible with a torch on ceilings. Look under evaporative airconditioning locations and areas that are under the roof valley flashings, these are common areas of moisture ingress.