We have a large quantity mix of fiber types in our network.
If the fiber is only a few years old as others have mention it is most likely 50u. With that said on the jacket should have at least have the manufacturer name and part number. I know what you are saying in that many jackets do not have a 50 or 62.5 stamped on it. Best is by name and part number and look that up.
The poor man's way to make an educated guess is one I have used many times over the years at many installations and is generally pretty accurate.
Most fiber installers are generally smart enough to match the patch cords to the fiber that has been pulled to the closet.
Here is one guideline to use if there truly isn't any manufacturer name and/or part number.
Color coding, If the patch cords and many times the jacketing are orange then there is a high possibly that it is 62.5 OM2 cabling. There are rare exceptions and that is usually done by a 3rd tier fiber manufacturer. Grey can also be 62.5, especially if it is the patch cords. You can technically run 10GB upto 80 meters on 62.5 but in a darken room you will see alot of light leaking around the connectors. Also will see a lot of crc errors under the show int status.
Aqua color for patch cord and jacketing of the fiber runs is almost always 50u OM3 & OM4. I doubt you have OM4 if it is several years old unless your employer paid the higher price at the time. About the same price now but not several years ago.
Doubt you have indoor single mode but if you do then generally it's yellow patch cords.
These suggestions are for indoor fiber. Outside plant can and in many cases are black jacketed.
Others many not agree but this has been my experience and I've done my share of indoor and outside plants when I worked for Ciena Communications selling fiber transmission gear.
I would highly suggest lifting some ceiling tiles and try to locate the manufacturer name and part number. That is going to be the most accurate way.