You are standing in the kitchen with a unit in mind, or a box already in the hall. Before you buy, walk the sink area with the cabinet doors open. A garbage disposal depends on three things under the counter: the sink opening, the drain path, and the power. Check each one before you commit.
The sink opening and the mounting
Look at the drain opening in the sink. Note whether the sink is a single bowl or a double bowl, and whether the flange is a common size. Most units mount with a ring that tightens from underneath, so you need enough room below the sink for the mounting hardware and your hands. Ask whether the mounting kit comes in the box or is sold separately.
Clearance under the sink
Measure from the sink bottom to the cabinet floor. A taller unit can hit a shelf or a trash bin. Check where the drain pipe will meet the unit's outlet, and whether the trap arm needs to move. If you plan to add a dishwasher later, confirm that the unit has the side inlet and the knockout plug for it before you commit.
Power and the switch
A disposal needs a circuit and a way to shut it off. Find out whether there is an outlet under the sink and whether a wall switch is close by. Some units come with a cord and plug. Others must be hardwired. Hardwiring is work for a licensed electrician, so plan for that cost and that visit before you choose the unit.
Questions to ask the seller
Ask who installs the unit and who answers if it has a problem early on. Ask what the warranty paper covers, how long it runs, and what you must do to make a claim. Keep the paper with your purchase record. Plain answers to these questions tell you more than the box does.
For the planning date, the typical service life of a garbage disposal is on its own page in the index: Garbage Disposal: typical service life and when to plan. Use it to decide when you will next look at the unit. It is a planning input, not a promise about any particular one.