What is your goal? Do you want to build it for yourself or friends or sell it as an idea? Do you want to start a full business or just make some for side-money?
Depending on your answer, good has different meanings... And tip #1 is essential to answer before you go any further.
Three possible answers make your product idea good:
Sell the idea! Stop. Nobody buys ideas.
Start a real business. Yep, step 2.
Make some for side-income? Okay, but tread carefully to step 2.
If this is a lark, stop now and proceed to hobby lobby!
Really, is there anyone who cares about your idea? Does it solve someone's problem or is otherwise desirable by some specific kind of customer?
If not, stop. Someone, some specific demographic, the smaller the better, must care, have the problem, or otherwise desire the product, or else, stop.
Can you sell one and for how much?
I don't mean build one then sell one.. I mean sell one!
The best way to know if your product idea is good, and the TOP TIP to know for sure if your idea is good is to PRE-SELL it!
Literally, setup a 2 page website where page 1 introduces the product, page 2 accepts a pre-order. If you can get a few pre-orders, go ahead a build *a few*...
You could also do this with Kickstarter or Indiegogo, but those services expect bigger ideas and a real budget for video/marketing/etc. Instead take the money you make on the first few orders to build some.... then build a few more... once you've sold 100, go ahead, do your kickstarter!
Go out and SELL! Sales is the only way to know if your product ideas is good or not.
Now, back to my Smart Watch idea....