Recently I returned from Thailand and Qatar. On my trip I noticed a wide variety of styles of packing. Some had many large and over stuffed suitcases and others had just a backpack only. Also a lot of this was based upon male/female packing style. So my question is this. How do you know if you have over packed or under packed? What is your preference when traveling, Backpack or Suitcase? How do you know what to choose?
As a traveler on a smaller budget, I tend to walk most places I go or use public transit over hailing a cab in a city. Because of that, I really prefer packing a backpack over a suitcase. I've seen one too many people try and haul a rolling suitcase across cobblestone to think that's a good idea. When deciding what to actually pack, I try and think of my own necessities (which tend to include two pairs of jeans, toiletries, etc) and then see how much space I have left for any "extras." Jewelry is always left at home for me and so is makeup.
You don't; you learn from experience - when you come back from a trip you think about if there were things you needed but didn't have/had to buy en-route, and if there were things in your luggage that you didn't use.
Generally though, and this won't help at all and makes this whole thing sound a bit 'trite', each trip is different for each person. I went to Israel/Jordan for two weeks and took only a laptop-bag sized backpack, whereas for my five days in a cold, wet Belgium I took a 60L rucksack; had I been going to, say, Everest Base Camp, I may have been tempted with something even bigger, because my needs would have been different. I will say though in general, you'd pack less for warmer destinations than colder ones.
The best way of realising you've overpacked is if your luggage is too heavy or awkward to carry. Conversely, you'll know if you've underpacked when you keep having to go to local shops and markets for essentials.
This also may be affected by how you travel - there's much more room on a coach/train than on a marshrutka or bicycle. Also, if you intend to do things like camping, obviously you'll be carrying more luggage (and being accepting of it) than if you stay in hotels/hostels all the time, but equally what's in your luggage will be different too.
For example, both me and a work colleague really like Slovenia, but if we spent a week there, I'd take a small backpack and she'd take two suitcases, because I'd be content with washing clothes in hostels and taking minibuses, whereas she'd rather be in 4* hotels, take taxis, and carry enough clothes to last - so she'd both have to and be able to take more and heavier luggage.
I tend to travel as light as possible, and haven't had hold luggage in aeroplanes for two years, but that's just my style - others need more and want more, so it is very much up to the individual. Only you can work out what you, yourself, need.
On my last trip I overpacked: 3 pairs of shoes (teva's, open and closed walking shoes in which I could put my inlaysoles). I had too much stuff to carry but that was because I had a big container with my tablet (which did not work, should have tried that at home!) and my breastpump. I should have taken one tectowel instead of two.
I swear by a small bottle of wildernis wash: shampoo, bodywash, laundry detergent and dishwashing solution in one! I love my Rick Steves handy laundry line, you can hang it on anything and don't need clothespins.
I missed lipbalm and did not buy it there because it was too expensive and was travelling on a light budget.
I agree that everyone's answer will be different depending on their particular needs and the destination.
Personally, I like to bring a carry-on sized suitcase (a Hey's hard-sided roller) and one 'personal' bag which is like an overnight bag. I've never felt like I'm too weighed down - but at the same time, I'm usually just visiting one destination.
I usually rent Airbnb properties for accommodations and try to get a place with at least a washing machine so I only need to pack about 3 outfits, wash and re-wear. In most cases, I've found that the host has any of the toiletries that I may have forgotten, but I try to pack light in that area as well (especially because most are liquid and need to be small and separate).
My goal is to get lighter and lighter within reason and I think for most people the issue is letting go of having to bring all of your 'things' with you or worrying about having everything for every situation!
You have only under packed if you have forgotten your passport.
Other things can be bought on location and you can live very fine without most things without suffering too much.
If I go to a place where it's a headache going from the airport to my hotel/wherever I am staying, then a backpack.
Overpack -- having to pay to bring your carryon or checkin bag.
Underpack -- not enough underwear to the point where you have to wash/dry used one :(
If you're strong and can carry around your 40 kg bag, then it doesn't matter how much or little you pack. Pack only what you can comfortably carry for your trip, whether in a backpack or a suitcase.
There are convertibles bags that act as both a backpack and suitcase for those that just cannot decide which is best (Osprey Convertible Packs).