I'm assuming this is geometry, not health, but here you go :)
The similarity theorems are foolproof methods for determining similarity in triangles.
SAS- where there's ONE angle in each triangle that is common, and TWO sides which are the same length.
ASA- where there are TWO angles that're similar, and ONE side that is common between both triangles.
SSS- where all THREE sides of a triangle are common between two triangles.
I hope this helped <3