Frontier Plumbing

You'll certainly want to ask them this. Steer clear of any company that is not insured in Nevada, because that means that any incidental damage they may do on your property will be a real hassle to recover compensation for. It is never wise to work with uninsured tradesmen of any kind. Most states also require that someone have insurance in order to be issued a trade license. If they say they're licensed but not insured, they're being less than truthful.

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 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.

Some companies may require a 10% deposit, some may not require any. For larger projects, you'll almost always need to give a deposit, and it's good to ask how much the deposit will be before going forward with a project. The Las Vegas contractor will know how much they need for a deposit when they've given you the estimate, so ask early.