core/src/main/java/com/orientechnologies/orient/core/index/OIndexes.java
public static Set<String> getIndexEngines() {
    final Set<String> engines = new HashSet<>();
    final Iterator<OIndexFactory> ite = getAllFactories();
    while (ite.hasNext()) {
      engines.addAll(ite.next().getAlgorithms());
    }
    return engines;
  }
public static Set<String> getIndexEngines() {
    final Set<String> engines = new HashSet<>();
    final Iterator<OIndexFactory> ite = getAllFactories();
    while (ite.hasNext()) {
      engines.addAll(ite.next().getAlgorithms());
    }
    return engines;
  }
Iterates on all factories and append all index engines.

@return Set of all index engines.