Ask about the physical workload of the job. Will you be required to handle materials? Lift or carry heavy loads? Assume awkward working postures with your back twisted or bent? Work with the arms overhead?
If the answer to any of these questions is "Yes" then find out how often this will happen. Try to get the possible employer to answer on a scale from "seldom" or "never" to "always" with "now and then" and "often" in between.
A job with "often" or "always" during a regular working day may be too much. Find out if you are allowed to ask for help from coworkers. Job strain can be reduced quite a bit with coworker support.
You can also be tested formally by a physical or occupational therapist. The therapist can tell you how much weight can be lifted and carried. The type of physical demands will be taken into consideration when making this assessment. Your weight, body type, strength, motion, and flexibility are also part of the picture.