Are 15A receptacles acceptable on a 20A circuit?
There is an exception in Code which permits the common 15A receptacle on 20A circuits if there are two or more sockets on the circuit. (NEC 210.21B3). The common double receptacle is two sockets. Mirroring that, there is a UL requirement that all 15A receptacles must have an internal capacity of 20A - only the socket itself is 15A. 

All the wire in a 20A circuit must be 12 AWG or larger. (Table 310.15(B)(16)).  If any of the wire is 14 AWG, then it is a 15A circuit with the wrong breaker on it.  12 AWG wire will not fit in the "backstabs" of 15A receptacles (NEC 110.3b), so if backstabs are in use, a Code violation is definitely present.  As a general rule it's best to avoid backstabs altogether, they are the #1 source of arcing failures.  (Code won't tell you that, but it does require AFCI breakers to compensate for backstab failures).

Assuming the circuit is correctly breakered, then the maximum load is 125% of the continuous loads (i.e. a 12A continuous load counts as 15A) plus 100% of the non-continuous loads.  This total should not exceed the breaker rating.  (NEC 210.20). 

A continuous load is expected to be on for 3 hours or more (NEC 100).  If all the loads are continuous, a 20A breaker can support 16A, and a 15A breaker can support 12A.  Otherwise break out the calculator.