Not exactly the most scientific but from the United time table for March we operate to the following out of IAD:

ALB, BDL, BTV, CLE, CMH, EWR, FAY, GRR, GSP, IND, JAX, MDT, ORF, RIC, ROA, ROC, SAV, SYR

A search through the United time tables for June shows we operate out of Dulles:

ALB, BDL, BTV, CHO, CLE, EWR, FAY, GRR, JAX, MDT, RIC, ROA, ROC, SCE, SDF.

If it’s somewhere new then you just gotta get lucky with a search, when it comes to this summer. March is the most recent month with a time table with an excel format available.