How can you tell? Start with patience, give them some time. You can candle them and see if you see any movement. But do not do anything dramatic even if you do not see movement. Just keep incubating hem and treat them as you would otherwise.
Or you can do what I would do, wait and see how many hatch. I don't think you have any problems at all from that episode. at that age the chicks are producing some heat on their own. What matters is the core temperature of the eggs. It takes them a long time to cool down enough to harm themselves.
I had a broody hen go back to the wrong nest. When I found her her eggs were really cold to the touch. I don't know how long she was off but probably all day. She hatched 11 out of 11 eggs. Plenty of other people on this forum have had incubators off for longer than that and still had great hatches. If you have any problems with your hatch it is probably not because of this.