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This will be answered when you ask if all employees are licensed. The only way to obtain a legitimate tradesman's license is to be a legal or naturalized citizen of the US. The company may employ people in positions that don't require a license, however, so it's wise to inquire.

This is very important, since an estimate is just that. Many contractors will submit a "paper estimate", meaning they've written the estimate down on paper. The bill is the true amount owed and will almost always say "Invoice" or "Bill" at the top of the page. The bill should also list the date it was issued, the due date of payment, and a list of services rendered.

You'll always want to ask how long the company has been in business in Houston. While this isn't a guarantee that the company will do good work, companies with longevity on their side must be doing something right. That being said, there are newer companies that do great work. If the company is under three years old, ask where the owner was employed before and for how long. If he/she has a long history of employment, it's reasonable to infer that he or she decided to go into business for themselves after so many years of experience.

This really only matters if you have a preference, but it is good to ask. If you have a strong ideological belief about unions or nonunions, it's perfectly acceptable to make a decision based on the preference.