How would I know that a compound and its isomers are enantiomers?
I am assuming that your question is this: 
"How to check whether two given compounds are enantiomers of each other?" 

First, you check for structural or geometrical isomerism. 
If there isn't any, you check whether the compounds are diastereomers. (Check whether they are not the mirror images of each other.)
If they are not diastereomers, you have two possibilities: 


Either the compounds are enantiomers of each other.  
Or they represent the same compound.


You could try this approach: 
First note the groups attached to the chirality centre. The Carbon is obviously an sp3 hybridized atom with four different groups attached to it. 
Interchanging any two groups attached to the chirality centre produces the chiral compound's enantiomer. 
So you could try interchanging the positions of the groups attached to the chirality centre until you get the other compound. If the number of swaps is even, the two compounds are identical. If odd, they are enantiomers of each other.