Current Output for ZVS Flyback Transformer
Secondary winding can give an idea. 

Measure the diameter of the secondary winding (d), then calculate its cross sectional area via \$A = \pi \ d^2/4\$. If secondary winding consists of paralleled multiple windings then you need to calculate total cross sectional area.

Maximum allowed continuous current can be approximated from \$I_M = J \cdot A\$, where \$J\$ is the current density with an assumption of \$J=420\ Amperes/cm^2\$. (Actually this \$J\$ value is valid for almost all SMPS power transformers as well).