There are various ways to measure how much fresh fruit and veg you're eating, but the easiest way – in my opinion – is to look at your hand: one serving is approximately equal to the size and thickness of the palm of your hand. You need to be a little creative in your thinking to get this one, but imagine three big juicy plum tomatoes (slightly squashed) – about equal in size and thickness to the palm of an adults' hand. The same goes for three plums, and also one average apple, banana or orange. The same goes for vegetables: chopped carrots, green beans, broccoli. Just think in three dimensions and you'll get there.
If you can't do that, just remember one simple rule: half of what you eat should be vegetables or fruit. Put the vegetables (potatoes don't count) on your plate first, and then add the proteins and starches.
And also – if you're going to achieve 5 a day, which is a minimum (Australia and the US have 9-10 portions as their recommended daily amount), you need to have fruit at breakfast and as snacks throughout the day.