A few aspects you must consider before picking a trek operator or guide are:
If they are well-acquainted with the area not only to guide you through the entire trail efficiently but also to determine the safe spots incase anything goes wrong.
Enquire the type of food provided by the operator. See if it matches your requirements.
Enquire if the trek guide or any of their staff has received first-aid training. It's imperative that someone from their team does. Also, ask them what kind of medical supplies they offer if anything was to happen.
Before you compare the prices offered by various operators and select the lowest price offering operator, make sure you go through the Inclusions of the packages offered. May be an operator is charging a tad bit extra but is providing better accommodation or end to end travel or an extra meal. Just check what exactly is offered at the price point and decide which one serves your needs the best. Remember, quality first.
It's always great to go with trekking companies that hire local staff whether it's in the form of guides, porters or homemade-food suppliers. This way, a share of your money will go directly to the locals.