How do I check my steel frame for water ingress, and minimise the risk of rust? I have a bottle cage on the upper side of my downtube, and yesterday I noticed that one of the screws has gone missing.  I don't know when this happened, but it could have been months ago.  I often leave my bike parked in the rain, so I'm now concerned that water has entered the downtube through the exposed screwhole and settled in the frame, which is made of steel.

How can I check if there is water ingress and minimise any damage?  My first thought is to turn the bike upside down for a few hours and let any water drain out, but is there a better way and/or more I could do?  I would like to stop any rust in its tracks.
Water will always find a way to enter the frame, if not through the bolt hole then via the seat tube or head tube or gas exhaust holes. Turning the bike upside down will not be very effective unless you fall into a river often. What you can do is 1) make sure that water has regular ways to leave the frame; 2) use special coatings on the inside surfaces to inhibit or prevent rust ; 3) perform regular maintainance to prevent bike parts from binding together. 

The lowest point of the frame is its bottom bracket shell. That is where all water tend to come. Check if the frame has a hole there so that water can drip out.

You will need to strip your frame before applying a rust inhibitor. See a good guide here: https://surlybikes.com/info_hole/spew/spew_care_and_feeding_of_your_steel_frame

Lastly, you should put grease at the places where the frame holds other parts: bottom bracket threads, seatpost, quill stem (if you have a threaded one ). That would prevent parts rusting together, making it impossible to detach them later.