Technically, when you have visible moisture and the surrounding temperature is below freezing (or the collecting surface) you have a chance of picking up ice. This does not necessarily mean that you will get ice since a certain moisture content is needed (you can relate this to relative humidity). And some clouds are purely ice crystals (e.g. cirrus), where the temp is always below zero, but you won`t pick up any ice there.
The best thing to do is to play it safe. If the temperature is around freezing and there is visible moisture, then you should stay away (provided you don`t have any de-icing equipment onboard). If it`s really cold, well, then it`s a different story, but again, the best thing to do is to play it safe - the good thing about weather is that it always changes
