or, if you do think your birth certificate will have it, you can always tell your parents, you are applying for a passport. you would need a copy of the birth certificate for that. if they ask you why you want a passport, just tell them you want it for a 2nd piece of photo identification. not necessarily that you want to travel abroad--or maybe you do want to travel abroad, or even go on a cruise, (you need the passport for most cruise tours also).
2ndly, I agree with Minderwiz. you can ask general questions of your parents, such as how long was your mom in labor for? did she have to go to the hospital in the middle of the night, or was it during the day, etc. lastly, if your folks had a baby book for you or if they sent out birth announcements, it may be listed there.
lastly, if not on the birth certificate, maybe on a town register--as some places here in the U.S. do require the birth to be registered, and sometimes will mark the time. not all birth certificates have the time. my husbands did not. then, again, you can also check notices put into the town paper too.
so, first off, why not just ask your folks what time of day were you born? if they ask why, just say you were wondering about it just out of pure interest.
there is another long drawn out way I believe you can use to try & determine how to figure out an approximation of time for your birth--but I think this is something that will take awhile. It would involve checking out the different times for the entire 24 hour day of your birth and follow it closely to see which one seems to describe yourself best. it may not be accurate, but about the best you can get if all else fails. good luck!