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They can lay eggs all year round and females do not need to have mated. They will need an egg laying area - we used a box hung from the side of a tank but this was for Musks which are smaller and lighter. Put some garden compost/soil in and hopefully she will climb in, dig and lay. You may need to set her up on her own as she may not be happy laying with the others around, although as you say they do lay them in the water but can become egg bound.
Hopefully she will lay them. Presumably you will not incubate them given the problem with so many Sliders and Cooters needing new homes all the time?
You can see when they have eggs and indeed feel them, but I wouldn't suggest you do this.
I looked at your previous posts to see which turtles you had as we have one or two!