It is more of a precaution then anything else. As long as they didn't get hot (warped) from not having grease in them and being pulled long distances they should be ok. Before you pull it bring a grease gun and pump them full of grease, If water comes out that means they where pulled through a flood and may need to be repacked. The best way to check is when you move the smoker pull for a couple of miles then jump out and touch the rims and hub. If too hot to keep you fingers on for any length of time they need to be serviced. Just don't pull the trailer if the hubs get hot, nothing good comes from it.
I wouldn't think they are sealed. If there is no grease connection on the hub all bets are off!