How to check if you can still get admitted to US?

There is no way to "check". Before they seek entry to the US again, the US government might not even know whether their visas have been automatically voided by operation of law or not, because the government might not have all the information needed to make that determination (e.g. the government might not know when they departed). Besides validity of the visa, it's also possible to have bans on entry to the US due to one's actions outside the US, which the US government does not know about until you try to enter. The determination of whether their visa is already void will only be made (after the fact) by the immigration officer at the port of entry when your parents seek to enter again. Before that, you can only look at the law and regulations to form your own determination.

The automatic voiding of visas due to overstay is provided in INA 222(g). Various manuals say that INA 222(g) is not triggered while one has a timely-filed, non-frivolous Extension of Stay or Change of Status application, which means that if she leaves the US before her EOS/COS is pending, her visa is not voided.

9 FAM 302.1-9(B)(1)(c)(4)(b) in the State Department's Foreign Affairs Manual says:

[...] In addition, if an alien departs after the date on the Form I-94 passes, but before his or her application for extension or change of status has been decided by USCIS, they must be subject to a blanket exemption from INA 222(g), if the application was filed in a "timely manner" and is "nonfrivolous" in nature. [...]

The CBP's old Inspector's Field Manual (they use a different manual now, but that one is not as publicly accessible; the IFM is still informative), chapter 15.15(g)(1) (page 138 on the page of this edited version, page 149 in the PDF) says:

Nonimmigrants admitted to the United States until a specific date who apply for E/S or C/S but who then leave the United States after the I-94 expires and before a decision on the application has been issued are not subject to section 222(g) of the Act if they can establish, to the satisfaction of the consular officer (if applying for a nonimmigrant visa), or to the satisfaction of the inspecting officer (if applying for admission at a POE) that they were in a period of stay authorized by the Attorney General prior to departure. The application must be timely, non-frivolous, and the alien must not have engaged in unauthorized employment, as provided in chapter 15.15(e)(2)(D). When these requirements have been met, the alien’s nonimmigrant visa should not be canceled.