What can I do to tell if my baby boy has svt?

Check the heart rate. The heart rate of a baby with SVT is much faster than a normal heart rate, often as high as 300 beats per minute. This rate is faster than you can count. Rather than trying to count the heart rate, i recommend feeling your baby's heart a few times a day to know what is typical. During SVT it is so much faster that most parents can easily tell the difference.
Why are you concerne. This is uncommon...Is he having rapid heart rates? Talk to his pediatrician if concerned.