Condensation is a very misunderstood thing you could in theory get it at less than 1°C under ambient, but it's unlikely you'd suffer all the factors that cause it to an extreme at the same time.
Here is a simple chart that shows the relationship between ambient temp, humidity, and the temp condensation is likely to start forming.
I've seen 10°C water temp on my system in a room ambient of 21°C and humidity of 50% and conensation was not an issue. To sum up If you are just water-cooling with a radiator and room temp coolant, (not using a peltier) and you don't live in the rain forrest condensation will not be a issue in your set-up. Leaks however are something you must check to be certian you haven't got.
*EDIT* for those that prefer it in °F
Convert Fahrenheit to-from Celcius