Detect

For the duration, you sense the presence of anything within 30 feet of you and weather or not it is good or evil. If you sense magic in this way, you can use your action to see a faint aura around any visible creature or object in the area that bears magic, and you learn its school of magic, if any. If creature does not have magic its aura will be either white or black depending on weather its of evil blood or not.

The spell can penetrate most barriers, unless enemy is using a magic spell to stay hidden.

Depending on how strong the aura is shows how strong the enemy is.