It is fairly simple to tell if the wind is adversely effecting your Slo-V once you've had experience. Basicly, the plane doesn't do what you want it to do and you need to constantly correct for wind. Maybe the easiest way to tell this when you don't have the exerience is once you've trimmed it for level flight and it flys 'hands off' on a windless day. After that, the more input you need to add to keep it flying straight and level, the more the wind is effecting your plane.

Sometimes it is also not how much wind there is, but how steady or turbulent it is. I've had my Slow Stick up in around 10 mph winds that were steady and non turbulent (almost like flying a kite when going into the wind). It is fairly easy to control with these winds (though not recommended for a beginner). I've also flown when the wind was almost calm, but gusted from various directions to +5 or so. This makes for interesting flying for planes with large wings with light wing loadings!

If you get stuck in a situation where you realize you should have stayed on the ground, as long as you have battery power, keep bringing it around for a landing approach and take the first one that looks like it will put the plane on the ground safe. If not, power up and go around again.