If a heavy particle A decays to two very lightweight particles B and C, what you need to do to find the mass of A is measure the energy E_B and E_C, and the angle theta_BC between the directions of motion of the two particles; then compute
2 E_B E_C (1 – cos theta_BC),
take the square root, and divide by the speed of light squared. The answer is called the invariant mass of particles B and C, and it equals the mass of particle A. Thus you can determine the mass of particle A simply by knowing E_B, E_C and theta_BC.
If particles B and C aren’t that lightweight, a more precise answer is to replace 2 E_B E_C (1 – cos theta_BC) with
(m_B)^2 c^4 + (m_C)^2 c^4 + 2 E_B E_C (1 – v_B v_C/c^2 cos theta_BC),
where m_B and v_B are the mass and velocity of particle B (and similarly for C), and c is the speed of light. Again the square root of this quantity, divided by c^2, is the invariant mass, and this will be the mass of the parent particle A.