how to determine if pants have enough room to let out at the waist

Look at the back of the inside of the pants. If there is a v shape getting wider towards the belt area then there might be room. Depending on the width of the v shape at the top will let you know how much more room in the waist there is to let out. The deeper the v the more room. If you measure the v it’ll give you a rough estimation on how much your pants waist size can increase. You have to remember that there will still need to be enough fabric left to stich up the new seam.