A/B testing is not always an option.

For example, when you create a new design, new user experience, new flow, you need to find usability problems first, before running any A/B tests.

There are 2 problems here:
1. What to check?
2. How many people you need to uncover usability problems.

1. What to check?
Here are 5 quality components you need to check:

1.1. Learnability:
How easy is it for users to accomplish basic tasks the first time they encounter the design?

1.2. Efficiency:
Once users have learned the design, how quickly can they perform tasks?

1.3. Memorability:
When users return to the design after a period of not using it, how easily can they re-establish proficiency?

1.4. Errors:
How many errors do users make, how severe are these errors, and how easily can they recover from the errors?

1.5. Satisfaction:
How pleasant is it to use the design?

2. How many people you need to uncover usability problems.

A research by Nielsen found that:
5 users find 85% of the usability problems.
10 additional users find just 15%.

So if you are tight on budget, 5 is more than enough.