You can look at Safeway/Trader Joe/Whole Foods prices and compare it to Instacart's to figure that out. That being said, do you ask Safeway/Trader Joe's/Whole Foods what their markup is? The only reason you care is because it's not completely hidden from you.
OTOH, Peapod does grocery delivery around here and just uses the local Stop & Shop's prices, including their weekly specials. I like it since I trust that a major chain will have competitive prices.