How do you tell if you got blood clots in the legs after a flight?

Pain and swelling. Commonly, blood clots cause pain and swelling but sometimes they will only cause one or the other. Occasionally, they cause no symptoms. Ultrasound is the only way to know for sure.
Venous doppler. If you develop calf pains after flying you should get a lower extremity venous doppler to make sure your do not have a blood clot.
DVT. Typically peaple get unilateral swelling. Localized swelling and pain is also possinle. In some cases shortness of breath which can be life threatening.