I am assuming you mean from a salon, or company that offers hair replacement service?
Before you ask the questions, make sure to get knowledgeable about Hair System lingo. Read posts here, and go here:
https://www.youtube.com/channel/UCY2VTiuzuax5HbyYvLLDu_A?sub_confirmation=1
This way, when they answer you, you know what they mean.
Do they offer a free consultation?
Are they full-service, meaning will they do the following:
1. Create a template
2. Take hair samples or match color
3. Discuss your options about the different base, or base combinations, based on your lifestyle, etc.
4. Discuss your options for hairline, density, direction, curl, proposed style, etc.
5. Offer to train you how to do your own service at home (detach, clean, attach).
Do they have a good, knowledgeable, experienced hair stylist who will do the cut-in?
Offer to show you before/after pics of their own clients?
Things to avoid:
Companies that want to lock you into a monthly subscription, like hair club.
Companies that don't explain how their systems are constructed, or the type of hair to use.
Companies that want you to order more than one system at a time, especially the first time, this is a HUGE no no! Nothing is more frustrating than paying for ONE system that needs tweaks, then paying for TWO that need tweaks. And, be prepared you may need to tweak your system for a myriad of reasons (color, density, hairline, length, etc).
And finally, understand that time is money. Buying from a salon is going to cost more, because they are going to invest time with you, especially in the beginning. Once you get going and are comfortable with it, you certainly can opt for trying less expensive options, like some of the more reputable internet-based operations.
Be picky, don't be in a rush. There are lots of people in this business (depending on where you live), the ultimate goal is to look good, and feel good about your choice.
Good luck!