In the interest of answering the question despite my not understanding why you would want to do this:
foreach (OneToManyRelationshipMetadata relationshipMetaData in primaryEntityMetadata.ManyToOneRelationships)
{
if (relationshipMetaData.ReferencingEntity == relatedEntity.LogicalName)
{
string relationshipName = relationshipMetaData.SchemaName;
//Do something with your relationship?
}
}
Where primaryEntityMetadata is of type EntityMetadata and ReferencingEntity is of type Entity. Then use similar logic for checking Many to One relationships.