The Toyota Previa is a popular car that holds its value well. It is a rare "mid-engine" car which presents some unique problems when doing basic maintenance like checking the oil.
Understand that the Toyota Previa is a "mid-engine" car so you will not find the oil dipstick where you would typically expect to find it. Go to the driver's side of the car and stand next to the driver's seat. Look underneath for a yellow tab. Pull the tab up with your left hand while pushing the back of the seat with your right hand. The seat will tilt back.
Note a rug piece. Remove it. Note the metal plate and the black knob. Twist it completely to unlock the plate cover. You may or may not feel the need to remove it.
Look for a yellow knob cover on the left. This is the oil dipstick. Take the dipstick out and wipe it off. Reinsert and then take the dipstick out again. Check where the fluid hits on the stick and whether or not it is low enough to require adding fluid.
If fluid needs to be added, then remove fluid cap next to the dipstick. Then use one-half of a can of ATF fluid at a time so as not to overflow the reserve and put undue pressure on the feed.
Replace the metal plate and twist the opposite way to tighten. Replace the carpet. Push the seat down until it clicks and locks into place. If the oil looks like sludge, it should be changed.