I have a GREAT system! Seriously...
I have a large hot pink teddy bear, about the size of an infant.
When I take the baby out of the car seat, I drop the teddy bear in the car seat. Or right next to it.
When I put the baby back in the seat, I immediately put the bear on the front passenger seat. [ I had to have 2 teddies because the babies usually wanted one nearby]
When you get to your destination, you have a hot pink teddy bear right next to you , as you pull in to park. It is impossible not to see it. If it's there, you know there is a baby in the car. I also have my purse in that seat so I am going to see the bear.