You can take a test, but if you get a negative and your periods still don't show up, please consult your gynac. They might suggest a blood test to be absolutely sure.
You can definitely test after 28 days, especially if that's your regular cycle length. Be gentle with yourself during this anxious wait, and use a good quality home test kit.
Yes, after 28 days is a good time to check. My mother always said to wait a couple of days after the missed date for a clear result, but you can definitely try now. If it's positive, do book a doctor's appointment.
Yes, you can take the test if your cycle is usually 28 days. But make sure to test with early morning urine for the most accurate result, and keep eating a balanced diet while you wait.