Very good question!

Some stuff you can tell just because you’ve tried on similar garments of “real” clothing in the past.

Otherwise, you have two options. The cheapest is to do what I saw the wonderful dangerous ladies doing a while back: Take a picture of yourself in either underwear or tight-fitting clothes and photoshop or doodle the character’s outfit on you. I’ve knocked plenty of stuff off my list because “woah, that looks AWFUL with my proportions!” this way.

The more expensive way is to do what you should almost always be doing with cosplay anyway: Make a mock-up of the outfit out of shitty muslin (if you work the coupons right for Jo-Ann’s, you can get 25 yards for less than 25 dollars, but this takes sales and planning). This doesn’t work for shit made out of spandex, but at the same time, it’ll let you work out fit problems BEFORE you make the costume with your expensive fabric, and it’ll let you know if you’re on the wrong track entirely.

There’s also some shit I do for really truly bizarre patterning in tight-fitting outfits involving turning myself into a duct tape mummy, drawing the outfit and the seam lines on the duct tape, and cutting this apart and using it as a basic pattern, but if you aren’t experienced at things like drafting facings and adding seam allowances yet, I’d recommend NOT doing that because it requires a partner and can occasionally backfire.

Hope that helps!!!