I know from cold hard experience (we all had to start somewhere)
that dreams can completely cloud the fact whether a product is actually useful or viable

most people actually start off with products and the unconscious notion that actually making and launching the product is so new and exciting that really deep down they dont even care if it succeeds or not. they just want to "make the product"

However the successful marketer will first look at whether it is viable, having been around the block and got over the 'novelty' of making a product. Now they prefer to only make a "successful" product

My team does sometimes team up with people with nice products to help bring em to market, and the funny thing is, that by talking to someone for 5 minutes, I can about 99% tell if they will have a success or not. (assuming our marketing magic intervention)

there are certain things to look for that include
- Do they want financial success? This seems like an easy one, but is not so cut and dry. The reality is most people don't WANT success. They want to try and learn and be comfortable in failing. We dont want people that say "at least I learned alot in the process"
- Are they trying to create "meaning" or would they prefer financial success. Again about 90% of new product creators out of the blocks are more interested (they dont know this) in creating "meaning" with their product, than doing the things necessary to have a strong financial payout.
-Is this product useful, original and can be positioned correctly? Yeah - seems like the first question someone should ask. But most creators don't even want to know the answers for this and just prefer to run blind.

There are infinite more things, but these are good places to start. However that being all said, I got to where I am by failing at all of the above points and learning what works then after the fact, so perhaps there is something to be said for the Prime Directive (for all you trekkies out there) or leaving people to their own karma and learning curve. . . .