I presume that you did this as a student and/or a volunteer and not as a paid worker. So, yes, it is harder to come by evidence that you put in the hours. That said, particularly if the end result was a publication with your name on it, no one is going to be tracking down the PI and confirming this. There are just not enough hours in the day for that level of detail. As long as your name is there and that checks out (looked up on Google Scholar or a similar search engine), you'll be fine.