Do you use RWGPS, Strava or other site to look at your ride data? If you know where on your ride you were doing an interval, you can select out that portion of the ride and look at it's data individually.
For RWGPS you click and drag your mouse pointer on the ride profile and watch where on the map that is.
For a trainer, I don't know what to tell you. Note the time since you started.
With a Garmin, you can just push the lap button when you begin and end, then it shows up in the upload. And if you are doing a planned workout, your Garmin will even tell you when to begin and when to end your interval automatically if that works for you.