Length & layering
Start with the outer garment
Lay the dress or skirt flat and compare the waist or shoulder starting point, hem, neckline and any side slit. A shorter base layer may be useful, but the right difference depends on the outfit and how it moves.
Three checks before leaving
Check the neckline in front of a mirror. Look at the hem while seated. Then view the outfit in daylight. Indoor lighting alone may not reveal how contrasting layers appear.
Lining is not a separate slip
A lining is built into the garment. A slip is removable and can be changed between outfits. Neither description guarantees opacity or a particular feel; examine the actual garment.