The excess could be seen on Form W-2, unless the excess was already removed. I recommend contacting the plan administrator for those details.
The 2019 contribution limits for 401(k) retirement plans is $19,000 for workers under 50 years old, $25,000 for those 50 and over. According to this IRS Issue Snapshot regarding consequences to participants who make excess deferrals to a 401(k) plan: The excess is taxed in the year contributed and taxed a second time when the deferrals are ultimately distributed from the plan. The excess plus any earnings must be removed by April 15 of the following year.