Also street signs. Most people in the US who drive are familiar with the shapes and colors of street signs. They're made uniquely so just in case the sign gets worn down and hasn't been replaced, you'd still know a yellow triangle means yield, or a red octagon means stop. European street signs are very different from ours and a dead giveaway.Often it Is the width of streets and sidewalks that give it away.
Also, one giveaway I always notice is the limestone. Growing up in the Joliet area, a lot of stuff around here and Chicago is built with limestone sourced from the Joliet-Lemont Formation. European limestone quarried from the Jura formation, the Tuffeau Formation, or other European limestones just have this different look about them. It's kinda one of those weird quirks of having a background in geology.