Chances are no one would know. I suppose (in some states at least) a police officer could check if they doubted it was a certified seat, but I doubt most could identify a foreign seat or would even care that much. Getting it here could be an issue, because the contents of the package have to be declared, and they do have the right to destroy items (with no compensation) entering into the US which do not meet US regulations. I have never heard of an issue with carseats, but I am certainly no expert on this. As far as I know, carseats are not a red flag issue (the way foods, medicines, etc are), so most likely no one is going to hassle with it at the border.
And, yes, it is mainly about the sticker. Most European seats would probably pass US testing standards (although some might not). But until they are tested, there is no real way of officially saying they would. There could be some minor differences which might cause some seats to fail, not to mention that the differences that exist might cause some parents to misuse them, because they are not going to read the manual well wnough to realize it needs a slightly different install than the US car seat they are used to. (I assume anyone willing to import a seat would care enough about car seats to check, but you never know.)