My ex pulled this, way back...tried to claim I kept an "unfit" house, and complained to CAS about it.
I obviously did not have an "unfit" house, so when the CAS worked talked to me about this (on the phone), she wanted to stop by for a chat, and I told her to come over anytime. Which she did - we agreed to a date for her to come over.
Most of the time, she was over, we sat and discussed how the kids are doing, and it was only at the end of that, that she basically wanted to do a quick "run through" the house. She basically just poked her head into each room, and this visit ended with a "obviously, there are no concerns here".
I think I would have been in my right too, to just refuse this "visit", but I had nothing to hide, and obviously wanted to show I'm the cooperative parent.
The end-result... I had to follow-up with CAS again, to find out anything, but I was advised there were no issues at my house, but apparently, they had talked to Mom, and it was advised to her, that she take a "parenting course", and apparently Mom agreed to that. But I don't know anything more than that, as I was no longer in the "file" I guess. So this could backfire on her.
You have nothing to worry about, unless you do keep an unclean house...then you should clean it. lol.
If you have clutter, or the kids have left their toys about...they won't care. They are used to seeing that. What they will look for is "dirt", mold, unsanitary conditions. Unsafe conditions.
My ex tried to claim I had a vicious pet, around our then baby...it was quite apparent that was not true, when the worker visited. If you have a pet, and it is not kid friendly, you should re-evaluate whether having that pet is wise.
I've heard, they look for food as well. I've heard, they want to see milk and bread available. lol. But maybe they look for those, because it's easy to spot spoiled food with them?
Agree with the other posters here... you need to thin your "friends", or associate with them less, if they hang out with your ex. There is "oh...I know your ex", and there is "yeah, your ex and I routinely talk". The saying is true... "some people do thrive on others drama". Those kind of people are not friends.