In and around Colombo the vast majority of tuktuks are on meters now, in fact don't get into one that hasn't or tells you it does not work as that can be a prelude to a ripoff. Elsewhere you need to agree price in advance use this for checking on the cost Fares Fair. The vast majority of these guys are honest but there is definitely a small proportion that will try it on.
I would suggest that for short journeys you just book individual vehicles from place to place, PickMe is the Sri Lankan version of Uber, once booked using the app on your phone the taxi is normally with you within 5 minutes. Cool thing is that you can watch the drivers arrival from your phone. I have added the website links to a few of the taxi services.