I'm guessing this is commercial.
I confirm sales data through every means available and then I call or email the broker with one or two focused questions. I always ask if the sale was arm's-length in their terms: unrelated parties, no duress, everyone negotiating hard. Those calls are well received because I'm calling with very specific questions.
Leases are much harder, IMO, to confirm. There are so many moving parts (TI's, free rent, escalators, renewals, term, start date, rented area) that the calls can take too long and no broker worth his salt is going to listen to too much of me digging. So, I get what I can before I call and do the best I can on the call. The attempted confirmation is valuable by itself. That's why offering memoranda are so valuable. Sometimes, you can't get much more than is given in MLS or CoStar.And data from other appraisers. Rent rolls from other jobs can help, so long as you follow your office policy on confidentiality.