As Jean-Karim suggests you should do cross-validation. In R you can use the CVlm function from DAAG package : http://math.furman.edu/~dcs/courses/math47/R/library/DAAG/html/CVlm.html
Anyway your post should be on the crossValidated (stats stack exchange) as it's not a bioinformatic question : https://stats.stackexchange.com/
These coefficients are only a measure of how well your model fits the data. They say nothing about the prediction accuracy. A common approach to measure accuracy of predictions is to use crossvalidation.