I've seen some methods for supposedly fully resetting the game so all the text is the same as it would be on your first time playing, which I guess is what you're supposed to do for speedruns. Is there a quick way of checking if it worked so you know the runs would be completely valid?

I think just knowing it worked once would be good enough overall, so I just finished a run, are there certain points in the Genocide route you should check on the text to make sure it all checks out? I did two runs, I know my first wouldn't have counted because I didn't do anything to reset all the information, and it was clearly obvious at the unskippable text at the end. But my second run I think that ending text was the same as the first time I did Genocide, the text at the end seemed about 20 seconds+ shorter than it was on my first run. Is that all the text you would need to confirm the reset worked? Or are there other points in the game to check, with differing text that makes a significant time difference?

Flowey's text at the beginning of the game would be one of the main telling points, as he can say a lot of different things if you haven't full reset and depending on where you last left your save off. I guess you could also pay attention to the Sans handshake in Snowdin too on whether or not you turn around before he says anything (and he'll comment on it). You'll know you've not reset properly if at the end of a Genocide run, the last text box before the final prompt isn't "LET'S ERASE THIS POINTLESS WORLD..."

There are probably some other good ways to tell so I'll leave that to anyone else to answer. Generally though so long as you've deleted the UNDERTALE folder in your Appdata > Local folder and don't have Steam Cloud Sync enabled, you'll have a fresh file. Just make sure to do this before every run.

uhh i bought it drm free is there a simple way? is true reseting not good enough? after pacifist wont you ALWAYS get the true reset button?

No, even if you do a "True Reset" in-game it'll still remember your past playthroughs. You actually need to delete the files (And the system_information_962/963 file after a Genocide run) for the game to "forget". You'll also need Steam Cloud disabled for the game since it'll automatically restore the system information file otherwise.