As far as I can tell, there are no "tools" to see your work in progress. It really sucks to spend 4-5 minutes loading/closing the game just to see 30 seconds of the game to see how it looks. Hopefully someone can find/make something for us painters.
Indeed, it really is a PITA 
now, you can also let the game run in the background in the hangar view, and load different liveries, it then lets you see updates...nut here it is quite noisy 
And this is only "acceptable" for top and sides views...
There are plenty of blender files available in User Content > Paintkits.
I use a fantastic one for the A320 which allows you to both paint direct onto the model in blender, great for applying logos across joints for example, but you can also instantly import the png files to view them in real time instead of loading the sim.
I thought I was late to the party in finding this out a few weeks ago but it's a game changer.
oki, no blender file for the F-15 I'm working on 