Investors interested in stocks from the Building Products - Retail sector have probably already heard of Tecnoglass (TGLS) and Fastenal (FAST). But which of these two companies is the best option for those looking for undervalued stocks? Let's take a closer look.There are plenty of strategies for discovering value stocks, but we have found that pairing a strong Zacks Rank with an impressive grade in the Value category of our Style Scores system produces the best returns. The proven Zacks Rank puts an emphasis on earnings estimates and estimate revisions, while our Style Scores work to identify stocks with specific traits.Right now, Tecnoglass is sporting a Zacks Rank of #2 (Buy), while Fastenal has a Zacks Rank of #3 (Hold). Investors should feel comfortable knowing that TGLS likely has seen a stronger improvement to its earnings outlook than FAST has recently. But this is only part of the picture for value investors.Value investors are also interested in a number of tried-and-true valuation metrics that help show when a company is undervalued at its current share price levels.The Value category of the Style Scores system identifies undervalued companies by looking at a number of key metrics. These include the long-favored P/E ratio, P/S ratio, earnings yield, cash flow per share, and a variety of other fundamentals that help us determine a company's fair value.TGLS currently has a forward P/E ratio of 8.83, while FAST has a forward P/E of 33.10. We also note that TGLS has a PEG ratio of 0.44. This popular figure is similar to the widely-used P/E ratio, but the PEG ratio also considers a company's expected EPS growth rate. FAST currently has a PEG ratio of 3.68.Another notable valuation metric for TGLS is its P/B ratio of 1.88. The P/B ratio pits a stock's market value against its book value, which is defined as total assets minus total liabilities. For comparison, FAST has a P/B of 9.71.These are just a few of the metrics contributing to TGLS's Value grade of A and FAST's Value grade of D.TGLS has seen stronger estimate revision activity and sports more attractive valuation metrics than FAST, so it seems like value investors will conclude that TGLS is the superior option right now.Want the latest recommendations from Zacks Investment Research? Today, you can download 7 Best Stocks for the Next 30 Days. Click to get this free report Tecnoglass Inc. (TGLS): Free Stock Analysis Report Fastenal Company (FAST): Free Stock Analysis Report To read this article on Zacks.com click here. Zacks Investment Research