How do you know when a shaving razor is still sharp enough?

I always have trouble telling when a disposable razor is sharp enough to keep for another shave. Is there a method/technique for this? I hate getting razor burn!|||You just have to test the razor to know if it's still sharp enough or time to pitch it. Just run it up your legs once, and if it hurts or cuts your leg then it's dull and get out a new one. If it still works fine, then there's no need to throw it out yet.|||todays razors mostly have a strip that fades/changes colour usually to white, when that happens you should change blade|||Hi I'm Pedro and I work for Gillette. Just saw your post and I thought I could help. You should change your blade at the first signs of bluntness, when you feel that it starts pulling your hair. Additionally, our blades have a blue strip that you can use as an indicator for when you need to change your blades. I hope this helps, but if you have any further queries, please get back to me on ukgillette.im@pg.com