Above is an example of poor side seam alterations done at a local alteration shop. The front doesn't match up with the back at top of garment. There are numerous puckers from trying to work in fulness without opening the side seam and doing it correctly. The lining panels don't match up. It is bulky. Thread color is obviously wrong.
Above is an example of correct side seam alterations. Everything is smooth and not bulky. It all matches up and thread color is correct. This alteration was done at my shop.
Above is an example of a horrible chiffon hem done at a local alteration shop. The top row shows wrong side of chiffon and bottom row is the right side. The raw edge has been serged with a white thread. Thread tension and stitch length are incorrect. The color of thread used is wrong. Chiffon flows when walking and the inside of the hem becomes very visible - meaning the wrong side should look as good as the right side of the hem.
Above is an example of my chiffon hem. The thread is correct color, the raw edge of the chiffon is folded under and the wrong side looks as nice as the right side.
So how do you know if your alteration shop is doing a good job? Take a picture of the seams before alterations and compare them to after alterations. If you can't see a difference, then it's a good job.
Do you even care about the quality? Expense of alterations can be very cheap if you don't really care about how it looks. If you are going to take the time to have your garment fitted, then I must assume that you care about the outcome.
It's very frustrating that some alteration shops do shoddy work under the guise of professional alterations. Whatever happened to taking pride in what you do?
I came across an excellent quote for anyone in the service industry:
"If you think a professional is expensive, wait til you hire an amateur." - unknown