Perhaps you should provide a little more information about the type of investigation that you are going to do. Is it an investigation for siting of a structure ? Is it for environmental investigation ? Is it a subsurface investigation for foundation design of structures ?

Visual aspects of site investigation should include anything that you see, which are critical for your design. In geotechnical investigations, you look for soil conditions, groundwater (ponding water, seepage outlets), bedrock outcrop (if any), etc.  For environmental investigations, you look for evidence of contaminations, etc. You may also look for existing ground surface features, such as creeks, grades, steep slopes, vegetations, thickness of topsoil, etc.

Assuming that you look for site/subsurface investigation, and you have time to read, you may visit the US Corps of Engineer's site for some engineering manuals
http://www.usace.army.mil/inet/usace-docs/eng-manuals/em.htm
The US Federal highway also has some design manuals, and can be accessed at http://www.wfl.fha.dot.gov/projects/pddm/

Most of textbooks on geotechnical engineering have chapters on how to conduct a site investigations, samples of borehole logs, etc. USBR's Earth Manual book is one of them.

Hope that this helps

ASTM has a standard for Phase I assessments entitled "Practice for Environmental Site Assessments: Phase I Assessment Process (E 1527-97)" which includes a good description of things to look for.  Besides, most banks (which require assessments prior to loaning on a commercial/industrial property) consider the ASTM standard as the appropriate tool for the job.  It is nationally recognized too.

I agree with the others--in general, when making a site visit, look for:
1.  Type of vegetation present (trees)
2.  Presence of surface water, ditches, etc.
3.  Topography (flat, rolling, etc.)
4.  Evidence of rock outcrop
5.  Type of exposed surface soils (clay, sand, silt, etc.)
6.  Type of nearby structures and their condition (pavements, buildings, cracked?  OK?)
7.  Evidence of dumping or filling
8.  Dead vegetation (could indicate contamination problem)
9.  Evidence of sinkhole activity
Most DOT's have manuals with forms for making site reconnaissance visits.  The FHWA and COE probably have one also.