One way to tell if a light-colored snake is blue is to simply observe it. Has it been out in the evening, or have you not seen the snake for several days? Also, babies shed roughly once a month. If it's been a month or longer, AND you haven't seen an active snake, you can bet the snake is blue.
It might seem like a regurge is not such a big deal. Yes, it is very frightening the first time it happens, but you will get plenty of advise on how to treat your snake after the regurge to make sure it recovers. But the cost of the regurge and recovery to the snake is great. I got two lavender clutchmates last summer. They weighed the same when they arrived. One of the sisters had a regurge when she was fed in the clear stage- she didn't look blue, but when I smelled the regurge a day later I found both the rotting mouse _and_ a shed. She underwent the regurge protocol- feed a mouse half the size of what she regurged, increase the prey size cautiously until she is back at the usual size for her weight, don't feed in blue. Seven months later, having made an easy, normal recovery with no further complications, she is half the size of her sister. 55 grams versus 110. So it _is_ a big deal even though you may not see the cost to the individual snake if you don't have an identical snake to compare it to. This is why I am very, very cautious about feeding while in blue.