You can convert a moneyline, spread line, etc into a probability % quite easily - this essentially is the % chance of winning that bet (which you are buying)
+250 = 100 / (100+ 250) ~ 28.57%
-250 = 250 / (100+250) ~ 71.43%
As long as you think the probability of winning your bet is better than the odds you're paying (or their underlying percentage), then that's a +Expected Value bet.
I like to equate the process of gambling with buying a nice new TV. You wouldn't just see a sweet flat screen and say "I want that now". You might do some research into whether said TV is going to offer you the quality you're paying for; whether the retailer you're buying from is offering a good price, etc.
The same is true for gambling. I like to make sure that when I'm betting, I'm buying odds at a cheaper price.