To fruit or not to fruit

At this time of year having enjoyed the blossom, I look carefully at the dying flowers to try and see if they are swelling up or not i.e. if they were fertilised or not. My plum blossom seems very dead but they usually look that way and then sometimes later there are fruit. The pears and apples and this nectarine (its a triple apricot, nectarine and peach so I may be wrong) look promising. Slight frost last night and its cool today.