Some of my ECs are difficult to verify: for one significant volunteer opportunity I was in contact with 1 person from the organization, but she left and I don’t have her contact info for verification. And another one is an internship at a university hospital, but my contacts are only with certain people as well, not the hospital overall, so if the colleges called, they wouldn’t know who I am. Some of my schools don’t have a letter of rec either so they can’t see if teachers reference them. Is this something to worry about?
They would also verify it with you. If you are being honest, there is nothing to worry about. AFAIK, rarely do colleges call to verify, but who knows, they just might have been doing so 
Based on the latest news involving families purchasing athletic spots for admission
http://talk.qa.collegeconfidential.com/college-admissions/2164305-u-s-colleges-step-up-admissions-spot-checks-after-scandal.html#latest
there will be more checking than before. Colleges are still limited in how many hours they can throw at that. IMO if they are just among a list of ECs you have then I doubt they will do more than a cursory check, if that. If the ECs show a great number of hours then there may be more due diligence involved.
Unless it’s something extraordinary and not mentioned in essays or recommendations, it’s highly unlikely they will verify.
But with the recent scandals, they do have a bit more of an eye on them, and some schools have said they are randomly spot checking.
The risk of having an acceptance rescinded, or being kicked out of school after admitted, stops most folks from making things up. As long as it’s accurate, you have nothing to worry about.
Just be honest - and tell your strongest story (not the time to hold back)