Regression statistics will typically include an R-squared value. The closer to 1 this is, the stronger the correlation between the returns of the two stocks.

This Month Published: 01 December 2015 Points of Significance Multiple linear regression Martin Krzywinski & Naomi Altman Nature Methods 12, 1103–1104 (2015) Cite this article ...

Michael P. Jones, Indicator and Stratification Methods for Missing Explanatory Variables in Multiple Linear Regression, Journal of the American Statistical Association, Vol. 91, No. 433 (Mar., 1996), ...