How much rain?
Are we talking an occasional drizzle or a good steady downpour?
I rode 50 miles of a century last October in a tropical storm that dumped 7" of rain and blew wind 30 mph. Many a bike were toasted after that. I had to remove my seat and seatpost and turn the bike upside down to drain the water out from the bottom bracket area. I had to remove the tires from the wheels and drain the wheels (!) - I have slightly aero wheels and water got in thru the hole drilled for the tube stem. Everything needed a good cleaning, re-greasing, and checkup. My bottom bracket was "toast" according to my LBS - but I cannot say with certainty that it was because of the ride in the rain. It had been giving me problems prior to that. Maybe the rain was the straw that broke the camel's back, so to speak.
So, if it was a lot of rain, be sure to give the bike a good shake and see if you can hear any water in the frame. If so, get it out! Otherwise, clean, lube and check per other suggestions here.