There is at least 50 factors to consider, not limited to:
1. Extension
2. Extensions Registered
3. Keywords
4. Industry or niche
5. Search volume
6. Potential end users
7. Cost per click
8. Trademarks (reason to avoid)
9. Existing websites
10. Existing traffic
11. Past sales
12. Current trends
13. Domain Length
14. Advertisers
etc...
There is no magic formula, it takes many many hours/years of research and market knowledge.
I would suggest following recent sales through Namebio.com and DNJournal.com, watching domain investor interviews on Domain Sherpa (YouTube), and watching auctions to see what domains sell for.
The DN academy course is a great starter for beginners to learn the basics of Domain Appraisals.