Its possible that if your dates are correct, you could take an early detection test like EPT. Make sure the test happens in the morning with the 1st urine of the day, as that will have a higher concentration of hormone if she's pregnant. If you get a negative, dont fret and test again in 5-7 days.
what is the best time to take the test. According to 28 day cycle her menses were due on 14th May
The only way to know, is for her to take a pregnancy test.