Start by looking at the BBO hand records. See the button "BBO hand records" on this page and follow instructions.
Go to the bottom of the page and look at IMPs won or Masterpoints won. If the IMPs won is negative, it will be hard to prove cheating. Similarly if the masterpoints are below 50, it will also be hard to prove cheating.
Ask me for the data on the pair. I will create two web sites. The first web site will have all the boards I have on this pair. Note that this may not be the same as all the boards you can access on BBO, I generally ignore Robot games etc. The second web site will contain only their leads. I create separate web sites for each pair, not for each player.
Players can cheat on the bidding, opening lead, and defense.
Human analysis is needed to detect cheating on the bidding. This is normally the most obvious (to a human) of cheating. It requires a large number of boards to review. It can take time.
The opening lead (OL) is checked in several ways. The first is to look at the double dummy opening lead accuracy rate, see explanation in the button "Terms" on this page. If the value is under 80%, they are unlikely to be cheating on opening leads. If the value is over 85%, this is suspicious. My software has additional checks on the opening lead to determine if you are likely to prove a cheating case based only on the opening lead analysis. This test requires a little manual oversight and can typically be done in 10-15 minutes.
The card play is more difficult for humans to analyse but easy for the computer.