You can always turn the garment inside out and pin or mark with chalk.  You will have to stand up straight to then see the actual fit.  However, there are many videos on youtube on how to make your own dress form.   I am about to make one myself as I have recently lost so much weight.  I am larger on top than on bottom so a store bought form will not work for me. You basically be in your underware, place a large black trashbag over you (cutting a head and armholes).   Have someone else begin to cover the black plastic back with duct tape.  when going down below your rump, not making it too tight on thighs.  Wrap with duct tape a couple layers thick to make sturdy.  Then have your helper cut a seam up the back being careful not to cut you.  Slip out of the form, and then a row of tape up the seam.     Once you have sealed it up, stuff it with news paper or old rags or Tshirts. You can now place it on a wood stand, the bottom of an old office chair, an old hat rack.    And it is EXACTLY your body shape. Hope this gives you some alternatives.