Start with the problem you are going to solve:

What is the problem?

Who experiences this problem and how often do they experience it?

Is the problem painful enough that people will pay for it?

How many of those people are able to pay, and how much are they willing to pay?

How are they currently solving this problem?

How much will it cost you to solve this problem? What is the potential revenue opportunity?

Example of a viable, stable business - locksmithing:

What is the problem? People get locked out of their homes, businesses, and cars.

Who experiences this problem and how often do they experience it? Most adults that own a home, business, or car will experience this once every six months or year.

Is this problem painful enough that people will pay for it? Yes.

How many of those people are able to pay, and how much are they willing to pay? Most working adults are able and willing to pay $100 to get into their home or car, especially if they are in a rush.

How are they currently solving this problem? Spare keys, friends and family with keys, AAA / tow companies, etc.

How much will it cost you to solve this problem? What is the potential revenue opportunity? Basic home locksmithing tools, certification, uniforms, insurance, business license, local advertising may cost a few thousand dollars, so breakeven might be 3-6 months.

There's a bunch of ambiguity encoded in to the term "viable" here. The number of viable businesses is almost infinite. Maybe think about restructuring the question with some contexts around that viability in terms of size, area of focus, type, etc, since this will inform the answer to the question heavily.

Focus all your energy on thinking sales: what can you sell, what're people looking to buy, distribution, sale cycle, are people actively buying, what will it take to sell, talk to the potential buyers. Then think who are you going to displace and how you will differentiate yourself.