You can usually open it and examine it, and listen to it and it will usually show any problems.
Its like buying used. I do it all the time. Open and look, and look all over it for cosmetic flaws, power it up and listen.
Nothing drops the value of a product as much as cosmetic flaws.
Car without a small dent = 25,000.
The same car with small dent = 6,000
I'll take the dented car, and run it for 300,000 miles, no problem.
If it isn't visibly damaged ( scratched cabinets are OK...lower price !) then used speakers are very good as they have been run in. Very unlikely that the drivers will be damaged with excess levels. You can play it and see.
Generally speaking..go for it !
If multiple drivers are there in speaker cabinet then check if individual drivers are working by just lightly touching the cone if u cannot make out by just hearing/seeing. For checking tweeters u have to go by hearing.
Sometimes damaged cones of the speakers get replaced by locally sourced one if original is not available. This was the feedback of manager of one leading speaker brand/ store in Delhi when he discouraged me to buy their display model. perhaps he wanted to sell the new ones. But i had told him my budget which meant i would not buy a new one. So check the cones of all drivers to see if they are uniform and original. Replaced cone can bring the price down if otherwise sound is ok.
Take somebody knowledgeable with u. Four ears are better than two