How to tell if a dog is at their correct weight?

This probably seems a simple question, but it's one that's stumping me at the moment. I know every dog is different, but I was wondering if anyone could give some general advice?

Rogan is a Rough Collie X, approx 1.3 years old, and his last vet check 6 months ago put him at 15kg. I continued to feed him AG Lamb&Rice, as he was used to it from the Dogs Trust and they recommended I feed 2 meals of 70g each day. Over time I've upped the amount to 2 lots of 80-90g and now he's less 'ribby'. He's long haired which makes it hard to see his shape, but his lower spine and hips can still be easily felt, though his ribs have filled out since I adopted him 6 months ago.

He gets fed at 10am and 8pm, with a biscuit at around midday. He sometimes gets scraps in his dinner feed, but rarely gets titbits. Any food for training usually comes out of his daily portions, and he has a pigs ear or rawhide bone once a week.

I understand that he's still young and gawky, but I can't help feeling he should maybe be fed more, though I'd hate for that to cause any problems for him. What do you think?

Eye and hands measure body condition not scales

http://vet.osu.edu/vmc/body-condition-scoring-chart

I look at the waist from above. If there is one, but it isn`t tucked in, I assume the dog`s OK.

That's lovely, thankyou. I do think he could stand to be fed a little more, he feels very 'awkward' under all his hair.

I also body score, all animals are individuals so it's largely down to any excess fat.
Obviously it's breed dependant as Greyhounds would be classed as emaciated

I prefer to stand over the dog (assuming it is a medium or large sized breed) with the dog standing upright, not sitting, between my legs. I run my hands across the rib cage lightly, not pressing. If I dog is too thin, you will be able to feel the ribs prominently. The ideal is that you can feel the outline of the ribs without having to press. Then I run my hand down the backbone, again you should just be able to feel it nicely. If it is very prominent then the dog is on the thin side.

I have found that Tai, now he is 10, is getting a bit of a belly on him - but he passes the rib and backbone test OK.

I was thinking Syd was getting a bit thin as he's getting older, he definately has quite a pronounced waist when viewed from above a tucked up tummy and I can feel his ribs easily. If you grab a handful all you get is skin. So I was thinking of upping his intake but before doing that thought I'd check what he weighed. So I walked him down to the vets to collect some meds for Rio that they didn't have in when we went for her appointment. Popped him on the scales and my thin dog that used to weigh 46kgs now weighs 51kgs and I'm f**ked if I can see where he's hiding 5kgs unless it's between the ears.

I always go on the 'horse way'! run your hands over the dog & should be able to feel the ribs, not see them.

It's like me Trouble, it's all muscle!! Cough cough