Here are some pointers to help with asssessing the potential of a growing site. If possible, it is always useful to talk to anyone who knows the site well - they may have information about the past use of the site and observations about light / access / other issues. You could bring a compass, tape measure and possibly a trowel for assessing the soil and collecting samples.

Site Assessment – key questions
What is the history of this site?
Has it been in use previously?
Can we find anyone with stories about the site?
How much light does the site receive?
How and when does the sun move across the site?
What items may cast shade?Is there easy access to water? 
Have crops been grown here before?
What was grown? What was successful?
Any 'indicators' – weeds etc?
Access issues?
What are other demands on the space?
Is the site level?
Shall we grow in or above the soil?
If below, what is the soil type and depth?
Are there issues around acess to the site?

Site Assessment - key considerations
Aspect (available light and warmth)
Shelter (windy sites can stunt growth, affect pollination and fruit set, impede growth of tall crops)
Soil depth (for good root growth, effective moisture and nutrient reservoir. A fertile well-drained loam of at least 30cm is ideal, pH 6.5-7)
Soil structure and texture
Shade from trees or other features
Microclimatic factors (frost pockets, wind) – can damage crops, shorten the ripening season.
Presence of perennial weeds
Inherent soil-borne disease or pest problems
Remains of former structures
Local water supply
Access