How can I know if a tree company is giving me good advice and a fair price?

Ask for an International Society of Arboriculture credential and ask if there are certified arborist doing the work or only doing the sales. ISA Certified Arborists are experienced professionals who have passed comprehensive exams covering all aspects of tree care.
Get it in writing. Reputable arborists have a detailed proposal and have their clients sign a contract. Read it carefully and ask questions.
Consider all the factors before accepting a bid. Examine the credentials and the written specifications of the firms that submitted the bids and determine the best combination of price, work to be done, skills and professionalism.
Ask for their safety plan and proof of insurance including Workman’s Compensation. A company which does not have Worker’s Comp insurance for all staff is leaving you at liability for a worker’s injuries on your property.
Check online reviews on Google, Next Door, and Yelp.