The idea about the soap is not bad, and has been used for years -- but it's easier to use WD40 because the engine likes it much better than soap and water.
About the "bent" valve - leakage may not be caused by a bent valve, it could just be some crud stuck to the valve holding it open. It could also be that the valve seat wasn't ground correctly from the factory and it's allowing air to flow past.
Basically, here's the only real way to check this -- put the cams back in, button up the engine and do a leak-down compression test. A local shop can do this and will probably charge you about $25 - $40 if the engine/bike is clean.
If they do find leakage - then you'll take the head off and use either soapy water or WD40 to check for valve leaks. Turn the head upside-down, so the dome is facing you. Spray around the valves with your stuff, shoot compressed air towards the back side of the valves (next to the springs). Look for bubbles. If you see 'em, then you know which valve is leaky/bad. Check all the valves, it may be more than one.
Remove the suspect valve (only one at a time). Visually inspect it for unusual wear patterns, check the seat also. put the valve stem on a known flat surface, or hold a straight edge against it - check for a bent valve - it won't be much, so look carefully.
If none of these checks show problems, put a tiny amount of Prussian Blue (just enough to color the entire rim) on the valve where it contacts the seat, drop the valve into the guide and rotate it gently - once or twice. Now remove the valve and look for ink on the seat - it should be coated in a complete circle - if it isn't, then the seat is the problem. It may have dropped (almost fallen out), or it could have a dent in it.
If you're not comfortable performing these checks, you can take it to a reputable Yamaha dealer and have them do it for you. The thing is, some shops don't do very good head work, so make sure a good mechanic will be doing the work.
Hope this helps you out.