The firs thing you want to note is that you never want to lose a Race in Street Racers 3D. Every Race that you win or lose is listed on your profile and is one of the criteria that you can use to compare yourself against other Street Racers 3D players. This means that your first priority will be to Race players that you absolutely know ahead of time that you can beat. To do this you have to look at their profile and see their player statistics.
When you are in the Races menu select a Street Racers 3 player and instead of hitting the Race button select the Details button. This will take you to that Street Racers 3D profile and here you can begin to analyze their record. First off, if they are at the same level as you but have Badges that indicate many more Quests completed than you did, then it is a good indication that they went through the levels quickly and are therefore ill prepared. If they have won dramatically more Races than they have lost, and have more Race Badges than you do then you may want to avoid them.
The best thing to look for is to see if they have fewer Cars, Upgrades and Real Estate than you. It is often hard to estimate how may of any individual item they have, but you can usually get a good indication if they are deficient. If they have a large Bounty Take Out count then it will show their proficiency at Races and should be avoided. Make sure you hold off on collecting Street Racers 3D bounties until you are much later in the game. This goes for placing Street Racers 3D bounties as well.