What they're looking for is what your home life is like. A clean living space, decently filled pantry/fridge, kids have their own space, age appropriate toys, kids are clean and look healthy, etc. They may observe you interacting with your son.
In theory, you don't have to let them in without a warrant or court order. But you don't really stand to gain anything by refusing entry. If you don't have anything to worry about, it's actually in your favor to let them in and see that you have a perfectly normal home. That way if someone else makes another complaint, CPS knows that there's someone out there probably using CPS to harass you. Not letting them, while within your rights, generally raises the CPS worker's suspicion.
They will interview you, and they will likely want to talk to your son separately from you, depending on his age.
Since the allegation was alcohol, unless they think you're under the influence at the time of the visit, they probably won't ask you to do a UA or breathalyzer. If the allegation was drugs, they probably would do a drug test.
They can ask. You don't have to submit. If you don't submit, they can try to get a court order to force you to take a test. They may also threaten to remove the child if they have reason to believe the child will be in danger if they stay in the home.
Bot***** *****ne: CPS without a court order or warrant has as much power as you let them have. But they do have the ability to get warrants and court orders and will do so if they feel like it's necessary.
There are good workers and bad workers out there. Most are really just doing their job making sure kids are okay. They don't want to ruin your life or hassle you. If an allegation is made, it's their duty to follow up on it.