look at the specs of what you want then google the component's for reviews / benchamark tests. if the laptop your looking at has for example an i5 8400 cpu. you'd most likely be able tofind a benchamark for that cpu on the internet. same goes for video cards. find the one your laptop has and then look at some benchmark reviews and stuff.

many review / benchmark sites allow comparisons so if you have a base spec your familier with you can compare against that. for a slightly better reference.