Everywhere I've driven in the US with HOV lanes, also has the restricted lane(s) separated from the unrestricted lanes by a single or double solid line and the lanes themselves have a diamond painted in them every X number of feet.
Even if you aren't able to read the entire sign, there are other clues that should alert you to the fact that you might not want to be in that lane.
The question of interest is not merely "is this an HOV lane" but "can I use it at this time" and "is there anything the sign says other than the usual stuff that I would need to know". You're supposed to read all road signs, and only after reading can you be sure that their information is redundant.
Yet HOV lanes can be minimum 2 people most of the time, minimum 4 people during certain hours. If you're in a car with 2 people, you want to know if you're allowed to use the HOV lane or not.
Even if you aren't able to read the entire sign, there are other clues that should alert you to the fact that you might not want to be in that lane.