How can you determine what gauge strings are actually on a guitar?
Not what the manufacturer recommendes but what might actually be on a used guitar that you purchase. How can you actually tell a .010 set from a .09 set?

Thanks

You could find out the thickness of a guitar string by using a micrometer on the unwound strings that is the 1st, 2nd, and the 3rd. The answer you get should point you in the direction of the gauge of the whole set. If the 3rd is a wound string then you have a pretty heavey gauge set.
If the guitar is second hand then you may find that an odd string has been used as a desperate replacement. again the micrometer test will give you clues to this.
I have been using 0,10 gauge sets for years because I find they are light enough for lead guitar work such as string bending. and also they are slightly easier to tune and they stay in tune for longer. They also seem to give a better sustain and greater tonnality. That is only my opinion based on the way I play.
I am about to buy a MIM strat this week an as such have been playing them in the shop. They have 0,09 gauge sets on them and at first I bent the string right out of tune and I found them a little bit more tricky to tune at first. So there is a difference in feel between 0,10s and 0,09s
I will keep to 0,09 fender bulits with my new strat.
All said and done its down to the player you are now and the player you will become.

A lot of guitar manufacturers use mostly 10-46 gauge strings on new guitars at least that what I find with Fender and Gibson/Epiphone.My personal preference is for 8-38 as I do heavy bending and my fingers aren't terribly strong despite 40+ yrs of playing.That being said string gauge choice is purely subjective.

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'65 Strat,65 Mustang,65 Jaguar,4 more Strats,3 vintage Vox guitars,5 Vox amps,'69 Bassman with a '68 2-15 Bassman cab,36 guitars total-15asst'd amps total,2 vintage '60s Hammond organs & a myriad of effects-with a few rare vintage ones.

I'd say if you are buying a used guitar and even a new one the first thing I would do is have a new set of strings put on. That way you know you have a fresh set and exactly what size they are.

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The time has gone, the song is over, thought I'd something more to say.