Experienced guitarists can and they have videos all over YouTube. Most common giveaway? Aggressive pick attack. Valve amps and modeling amps respond in subtly different ways. They also respond differently to volume - that's important for guitarists who set their amp near the breaking point and use the volume pot to control their tone.
Having said that, for the average bedroom/small gig player these subtle differences are too minor to bother with. Especially when a modeling amp can accurately emulate so many different valve amps. Sounds good and you can save a lot of money!
BUT - I would be remiss to say that you shouldn't own at least one valve amp. Take the one you emulate the most and buy it. You'll be happy!
Having said that, for the average bedroom/small gig player these subtle differences are too minor to bother with. Especially when a modeling amp can accurately emulate so many different valve amps. Sounds good and you can save a lot of money!
BUT - I would be remiss to say that you shouldn't own at least one valve amp. Take the one you emulate the most and buy it. You'll be happy!