There are a lot of questions that you can ask, but you definitely want to make sure that the press delivers time, temperature and pressure accurately.
Be sure to look at the top heat platen - hold a straight ruler up to it, on edge, from corner to corner to make sure it is not warped at all. An uneven heating surface will not apply pressure and heat accurately on all parts of your designs compromising their durability.
Also, ask to see the heating element that is inside of the top platen. You will want the heating element to be evenly spaced throughout the platen and make sure enough element is used. A simple "Z" pattern will not suffice. This creates cold spots, so the element should zigzag throughout.
Also, make sure the pressure is adjustable to accomodate different thicknesses of items.
And check the warranty.
There are many more things to ask and below is an almost exhaustive list in case you're interested.
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The first decision you need to make is between the style of the press (swing/clam/draw) & the size - and then try some of these questions.
Time: - Does the press have a timer?
- Is it a digital timer?
- How many seconds can the timer be set for?
- Will this accomodate my applications?
- Are there dual time settings?
- Does the press beep, buzz, open automatically when the time expires?
Temperature: - Is there a temperature gauge?
- Is the temperature digital?
- What is the degree variance on the temperature readout to the actual temperature of the platen?
- Is temperature communicated using a bimetallic thermometer (5-10 degree variance) or an RTD probe (to the degree accuracy)?
- How high does the temperature go?
- How long will the press take to heat up?
- Is there heating element throughout the upper platen?
- If so what is the spacing?
- What is the warranty on the heating element/guarantee?
Pressure: - Is there a pressure adjustment?
- Is it digital? If not, how is it adjusted/measured?
- Is the pressure over the center?
- Can the highest pressure accomodate my type of application?
- Is there even pressure on every corner of the machine?
- Is the top platen stress relieved to ensure an even heating surface?
- What is the density on the bottom pad? Will this provide for an even pressure?
Other items to consider: - How thick of an item can the press accomodate?
- What is the warranty on parts & labor?
- What is the warranty on electronics?
- Is it UL and/or CE approved?
- What is the price?
- Is it easy to lock down and/or open?
- Is it 110 or 220v?
- Can I plug it into a standard household outlet?
- How much electricity does it pull?
- How many amps?
- How much does the press weigh(portability reasons)?
- Does it have interchangeable lower platens?
- What is the top reason that people choose this press?
- What is the biggest complaint of owners of this press?
- Will you ship it for free
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Buying new heat press -sketchy? 
