a) I guess it depends on the kind of scare and b) you never know until it is too late.
If it's just a basic reflex thing ("boo", "rolling eyes" etc.) with not much meaning => then fine. Most kids like to do it themselves! Note that you can pretty much do those things while being truly funny, not scary (i.e. by getting into it slowly, by having the kids do it along with you and so on). At around age 5-8 or so, children learn to disassociate our actions from reality; i.e. that's the time where they learn about jokes and all such things that are not quite true. Check for signs that they know that you are not actually evil when scaring them in this way, and you'll be all right. If you enter the room, do they tend to hide behind their parents or come running at you? That should tell you everything.
On the other end of the spectrum, what obviously does traumatize them is phsychological scare of the kind that you normally would not inflict casually. Like frequently hearing the parents shouting at each other, with one or both of them threatening with leaving, or worse. Or being left alone at very young ages while not being mobile yet, for long hours every day. Let's not even get into wars. That is not what you are asking though.
Borderline would be ghost stories. I recall some uncle telling really gritty mystery stories once while we were camping somewhere in the woods (me age 10 or so). I definitely had bouts of not liking being alone in dark places so much after that (but got over it). It depends how strong the kid's imagination is, and how good they are at telling a story/sarcasm/irony from reality. They say that that's not a given until 8-9 years old.
That said, I personally make it a point to not lie to or spook children at all, I don't use irony/sarcasm or the more rough "humour / stupid-talk" many adults use amongst themselves (and sure, I do engage in these with adult friends). All children I met so far (my own and others) were very appreciative of being treated earnestly. The last thing I want to do with them is to give them yet another adrenaline spike... they get that plenty from friends, school, older children etc.
Last point: as a parent myself, I would be very unhappy if someone spooked, tickled or pulled my childrens legs. Just saying. If the parents you are involved with don't mind then fine, but... really, I would not make it a habbit with all children. Are the children missing out if you stop doing it? If not, and you are obviously are not sure if it's OK to do it, then that would be a clear sign to stop.