Trace of Frobenius endomorphism of an elliptic curve with complex multiplication

Let E be an elliptic curve with complex multiplication by R, where R is an order of discriminant D < -4 of an imaginary quadratic field K. If a prime number p is decomposed completely in the ring class field associated with R, then E has good reduction at a prime ideal p of K dividing p and there exist positive integers u and v such that 4p = u(2) - Dv(2). It is well known that the absolute value of the trace a(p) of the Frobenius endomorphism of the reduction of E modulo p is equal to u. We determine whether a(p) = u or a(p) = -u in the case where the class number of R is 2 or 3 and D is divisible by 3,4 or 5.