Thank you for asking your query at DoctorSpring.
There could be a lot reasons for a delayed periods. But in a sexually active couple the first and foremost reason could be pregnancy. There is only one way to know it for sure which is a Urine Pregnancy Test. I recommend you to get a UPT (Urine Pregnancy Test ) kit from a medical shop and carry out the test (following the instructions in the kit) and let me know the results.
There is no other easy way to know for sure whether she is conceived r not.
Hope this helps