How can an investor know if a startup issues a SAFE to another investor with a different valuation? How can the investor be aware of it or track it?

We recommend the investor add a Follow-On Investment Information Rights side letter to the SAFE. This would be something like: 
While the Investor’s Safe remains in force, the Investor shall have the right to receive copies of subsequent Safe or convertible debt instruments executed by the Company at any future date. The Company will provide such copies physically by mail or by electronic mail within 10 days of execution to the address(es) provided.

An investor may be able to find out if a startup starts issuing SAFEs to other investors with a different valuation, but this will largely depend on the investor's relationship to the startup's founders, as well as the startup's policies on updating their investors. Following up with the startup in accordance with the investor's/Fund's valuation policy is also important. 

However, there are generally no obligations for a startup to tell an investor whenever they issue a SAFE to another investor, regardless of valuation. The same may be not true after a preferred stock financing, where other types of notification obligations may come into play.