Ask questions! You want to be sure the professional you hire is capable of meeting your needs. Talk about their most recent events. Ask what obstacles occurred and how they resolved it.
If they insist everything was perfect, you might want to keep looking. (Something unplanned ALWAYS pops up or we wouldn't be such a necessity.) They either weren't observant enough to be aware of the incident or they don't want to tell you about it. Neither is a good sign!
If they're not willing to share information openly during the interview process, how can you expect them to be clear and upfront while planning your greatest lifetime celebration?
An experienced planner will have an encyclopedia of "OMG moments" and be more than happy to tell you about how they managed to pull of an outstanding event anyway. (If for no other reason than because our friends and family are tired of hearing those stories.) This is a time you need someone who is willing to tell you what part of your plan has potential to fall apart. Not to say don't do it, but to at least be prepared with a back-up plan. Someone who nods and says nothing can go wrong is definitely not going to be an asset when something does.
Bottom line, you should feel comfortable that your planner will know how to head off adversities before they ever become visible to you or your guests.