Good advise from calipoppy. Drive around the area. Especially on weekend nights. Listen for loud parties. Check out the action on Fair Oaks Ave. See where the riffraff hang out. Check very carefully for gang grafitti and signs of grafitti removal on street signs, curbs, bus stops, walls and businesses.
Don't be afraid to stop and approach a future neighbor working in the front yard. Ask them questions about the area, schools crime, homeless and transients.
Check out this website scroll down to the map and see if any of your future neighbors have been recently arrested.
Altadena Crime News - Free Arrest Records in Altadena
Check out the Megan's law website for sexual predators and their location
Look for abandoned shopping carts. Junk cars parked on the street.
If you see people walking their dogs and strollers during the evening, that's a good sign that residents feel safe. Try to get a handle on the next door neighbors both sides and back. A neighbor from hell can trump a lot of good data.
Why did the former owners move?
This is a big investment and judging by the economy, you will be living there for a long time. Altadena can be a very nice community.