It's not always easy to spot a real Edomae sushi chef in the US, or even in Japan. One sure way to do it is by looking to see if they use oboro in the preparation of certain nigiri. If they do, that's a pretty good indicator that they adhere to Edomae preparation standards and methods and go the extra mile to produce great tasting sushi.
Oboro is crumbled sweet shrimp paste. Shiba-ebi is mixed into a paste using a food processor and/or mortar, then sugar, sake, and a touch of salt is added, and the mixture is briefly cooked. The resulting paste is placed between the shari and neta or directly on top of the neta.
Oboro, due to its ingredients, is very complex in flavors. It is predominantly sweet (from both the sugar and the shrimp) but also a little salty and umami. It adds a lot of complexity to nigiri, especially to pieces like kohada that have been marinated in vinegar. You'll first taste the sourness of the rice and marinated fish, followed by umami, then finally the sweetness of the oboro will come to the forefront, leaving you with a sweet aftertaste.
Although considered an important ingredient in Edomae-zushi, many chefs no longer bother using oboro in their nigiri preparation these days, which is a real shame.