STEP ONE: Look for droppings. There will also be brown grease in the entrance point. It is usually easy to determine where they get in and out.
STEP TWO: View the home at dusk. Even if you believe you know all the bat entry/exit holes, watch the house at dusk at least once. You will often get a surprise: how many bats could be different than you expect, but more importantly, they might be coming from areas that you did not suspect. I've learned that it is important to do this dusk watch.
STEP THREE: as soon as you know the circumstance, quote a price. Quotation to seal, or "bat exclusion", the whole house. Inform the resident that in the event you do not, the bats will come across a new area of the home, which you only guarantee what you seal. Charge appropriately to bat proof the entire home - it is very detail oriented work. Also quote a cost to eliminate all the bats via appropriate exclusion netting. It is possible to make this cost any amount you feel is reasonable, depending on the location and difficulty of installation. Also quote a cost to clean and decontaminate the droppings in the attic or soffits, and show the client photos you have taken in their attic of this bat waste. If it's a huge mess to clean, and a lengthy project, the purchase price will be higher.
STEP FOUR: You have got to do this 100% flawlessly. Do not leave any gap available, anywhere. Bats can get into amazingly small gaps. You may schedule to perform this sealing throughout the day, but I really like to do it at night for two main reasons: first, the majority of the bats will be outdoors, meaning that should you inadvertently seal an entry/exit hole, then you won't seal many in. And second, you can see the openings better at night with a headlamp than during the day in the bright sunlight and shadows.
STEP FIVE: I like to do this an hour after dusk, because the majority of the bats are out anyway, meaning that if you mess up somehow, it is not as big a deal. However, it is best if you do it absolutely correctly, so you never seal in one bat. Here is the principle: you need to drape a net over the hole in this manner than bats can flutter from the hole and fall down and fall from the open base of the net. However, it has to block them from flying back into the hole. Bats are creatures of habit when it comes to returning to the entrance hole, they will need to land a particular exact way, and they can not hover. So when they attempt to fly back , they will fly straight at the hole - they do not know enough to fly under the open bottom of the net, and their flight ability probably won't let them anyhow.
STEP SIX: If you are working far away from home, you can drape the net an hour after dark and then remain in that city overnight and remove it the following day and you will be nearly, if not entirely bat-free. When it's in your town and driving concerns are not an issue, allow the net stay for 2-3 days just to be absolutely sure.
STEP SEVEN: If you have sold attic decon, do it after the bats are out, obviously. You can have a little battery powered vacuum to the attic and vacuum up the droppings, and you may spray/fog the place with Bac-Azap in an atomizer, using the appropriate protective equipment and filter mask.