The textbook "CR=ECL" situation is supposed to use up 20% of the party's resources. That includes hit points, so they won't usually result in a PC death because each character represents -on average- 25% of the party's HP.
I would look at the enemy/creature's stats and see:
1) How often -statistically- they would hit the character given the character's AC and the enemy's BAB.
2) How often the PC would hit the monster (inverse of above)
3) How much damage they can do to each other per attack compared to their total HP.
4) Make sure that any special abilities (Fear, Poison, etc.) are not so high a DC as to make it nearly impossible for the PC to survive.
Basically be sure the PC can hit at least 30-50% of the time, can take a couple hits (barring criticals) before dropping, and can drop his foe within close to the same number of hits he can take himself. Maybe 1-2 less.
Be very careful of multiple opponents. Unless s/he can drop one per round, even weak creatures can give a lone adventurer a lot of grief.