Would you see that 1.25 as a value bet or not?
What if a tennis player in Wimbledon or some other big tournament, quickly goes 40-0 and you have him @ 1.10 to win the game (not the match, just that 40-0 game) - can this be a value bet?
What about a basketball match, the total line is 180.5 and the game in live goes 150.5 - is it worth taking the "over" because the line has dropped 30 points below the original line?
How do you define value in general?
Unless you're crunching huge amounts of information like the casinos then at best you'll end up making an "educated guess". If you're keeping track of your betting and doing analysis, maybe over a few years you could determine that your judgements on value are accurate - because otherwise variance might bring your profit back down to zero on short timescales. Honestly you'd think things like a team being two goals up in a football match with 5 minutes to go a sure fire winner, but I've seen enough reversals of fortune to know it's not a long term winner.