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Join Date: Jun 2013
Location: Cedar Rapids, IA
Posts: 1,643
Bikes: 1997 Rivendell Road Standard 650b conversion (tourer), 1988 Schwinn Project KOM-10 (gravel/tour), 2013 Foundry Auger disc (CX/gravel), 2016 Cannondale Fat CAAD 2 (MTB/winter), 2011 Cannondale Flash 29er Lefty (trail MTB)
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You could figure it out by letting all the air out, and removing the lock nut on the valve stem. You'll be able to tell if there's a tube attached to the valve without breaking the tire bead.
Not sure on sealants. Search on this site, MTBR, and elsewhere for reviews.
I know of folks that zealously defend Stan's, Orange Seal, Caffeelatex, or homebrew sealant as the absolute best. Slime has a bad rep from their green slime-in-a-tube days, so I know a lot of folks want to hate on their tubeless recipe even if it is OK.
Personally, I've used Stan's and now the Bontrager stuff. My girlfriend works at a Trek shop and brought a bottle home, and it seems to work really well. I like it better than Stan's.