Whether you are making a subjective or data based assessment of the skill sets of footballers the approach is similar in both cases.
Has the player out performed a nominally chosen benchmark figure for the attribute you are measuring.
For example, if a keeper makes saves that an experienced observer considers exceptional or if he saves more attempts than expected by a statistical model based on the average performance of his peers.
The limitations of such observational based evaluation lies in sample sizes. Keepers may produce hot performances that inevitably cool and aren't indicative of the general level of performance and levels of good or bad fortune may be present in any data set.
I've 67 on target attempts faced by Fraser Forster in 2015/16, he conceded 17 goals and simulating these shots faced can show how often an average keeper, represented by an expected goals model, would have conceded as many or fewer than Forster did that season.
Forster's 17 goals conceded is equalled or bettered by an average keeper in around 24% of trials and is represented by the orange part of the distribution.
While an over achievement is obviously desirable, simulating all attempts also adds information about how likely it was this over achievement occurred by chance. Forster may not maintain these levels, but he may be a consistently above average shot stopper.
The table below uses shot data from 2015/16 to see how often an "average keeper" simulation of the actual attempts faced by Premier League keepers resulted in the par keeper equalling or bettering the actual number of goals conceded in reality by each keeper.
Only 11% of simulations managed to equal or best Fabianski, whereas at the opposite end of the table Stoke's keeper crisis in the continued injury absence of Jack Butland is starkly revealed.
Their current first choice keeper is 40 year old Shay Given, who is unsurprisingly injury prone and his performance in limited appearances last season was equalled or bettered by 80% of average Premier League goal keeping prowess.
Jakob Haugaard, his younger and initially preferred counterpart, raises that under performance to include every average keeping simulation.
Haugaard conceded 9 goals from 18 on target attempts against a cumulative expectation of just over three and I've yet to find an iteration of those 18 attempts that does worse than the young Danes' unimpressive introduction to Premier League duty.
Shot saving is off course one of many abilities demanded of goalkeepers by modern day managers, but it remains a substantial contributing factor to their valuation and age is also a factor. Even keepers have ageing curves, albeit right shifted compared to the overall footballing sample.
These two simple inputs of age and likelihood of over performance from last season broadly correlate to each keepers current valuation on Transfermarkt.
Courtois and de Gea's valuations, rightly or wrongly have made the jump to vie with the inflated valuations of mainstream attacking players, but the input of the two variables listed above generally identifies those who command a high price amongst their peers (even if their over performance is likely to regress) and those who risk being replaced by a loan signing from the Championship (even if their parlous under performance is also likely to become less extreme).