I noticed there are some posts on here in regards to not getting ripped off when getting your vehicle detailed. If you are unsure of their work, ask about what their policy is about a warranty and refunds. Around where I live shops will offer a 7-30 day warranty.

You have to watch out for detailers, excpecially unhappy ones that are making minimum wage/commision and with no benefits. They are masters of deception and I've seen just about every trick they have. I know, I worked with one. Also, you must take into consideration that cleaners see many vehicle a day and experience deja vu where the think that they already cleaned a certain area of your vevicle. You will know just by the first look at a vehicle after it's detailed if you have a good clean vehicle on your hands.

BEFORE you leave the shop that cleans your car, inspect it INSIDE AND OUT, INSIDE AND OUT.

Now remember, be fair, if you pay for a wash, vac, and window, you aren't getting a wax and polish along with it.

Also remember, check inside and out, inside and out (You'll find the meaning of that in a minute).

Now while the car is parked outside:

Check visually for any residue left on the vehicle, in cracks, door/hood/trunk jambs, wheel wells, ect. Make sure you get them to correct the wax that has found it's way onto a textured surface, easy to spot, it usually turns it white. Make sure all polish/acid/brake dust/tar is removed from rims.
Then, run your fingertips along the bottom half foot of the door checking for any road tar and visually inspect for brake dust.
Make sure you inspect around all badging on the outside of the vehicle to make sure there in no wax in any of the crevaces. While checking on the tires and rims, take a look in through the wheel wells to ensure they have been cleaned.
Look at all of the windows in all corners making sure the windows are clear. Look hard, you may find some streaks.

Now on the inside, be fair and start from the bottom and work your way up.

Make sure carpets are clean, even some of the toughest dirt stains come out with some rubbing with a rag and multi-purpose.
Check doors panels and inside any cupholders and door handles to make sure they are clean of gum/coffee/dirt. Also look at arm rests and head rests to make sure everything is cleaned.
Fold the seats down (towards the back of the vehicle) and check for and crumbs/lint/dirt. Make sure you look under the front seats and any back seats that fold up, they are easy for a cleaner to miss.
Now turn your attention to the dash and any other compartment that is in the vehicle. Look at panels that are down by feet and all around the glove compartment when open. Look around vents and switches for anything that may be there. Don't forget about the rear deck of your cars.
Then you should start looking at the pillars and windows (checking all corners)and then turn your attention you the headliner.

Now the inside and out, inside and out. You already did one inside and out, outside, now ask an employee to take the car inside so you can check it over again.

Go through all of the areas mentioned above, but do it inside a washbay where lint and other little particles are eaier to see when there is less light.

Make sure you wash your car 4-5 days after it was detailed to see if the water still beads on your paint, if not, they used a cheap wax. Go and get it fixed by the shop.

If you find problems, let somebody know so they can be corrected, if they don't fix them, take their vacum cleaner.