Request and review the adoption agreement or employee policies and procedures manual. You can also ask in the HR dept of it is available in the plan. Knowledge is power!
The best way is to get a copy of the plan summary document directly from your client, or the HR department if the client has misplaced it. Alternately, the client can call HR with that direct question. In most cases, if an employee is over age 59 1/2, he or she is able to take an in-service distribution. I've seen some plans that limit this to only rollover amounts in the plan. The plan document and HR are your best resource. Good luck.