The Challenger 300 series is essentially a scaled-down version of the Challenger 600 series, so there are similarities. It remains an aircraft with low mounted, swept wings, a T-tail and engines attached to the rear fuselage on pylons. The nacelles have a single exhaust. The cockpit windows, which become narrower towards the end/side, are a typical recognistion point. The vertical stabiliser has a short curved dorsal fin with an air intake at the root.

To recognise the different versions of the BD-100 family you will have to look at the shape of the winglets.

The first version of the BD-100 has winglets attached almost perpendicular to the wings. This is the only external difference compared to the Challenger 350 and 3500.

The second and third iteration of the super mid-size Challenger have winglets attached to the wings at an angle of about 45 degrees. These canted winglets are a bit larger than on the Challenger 300. This is the only external difference compared to the Challenger 300.

The Challenger 3500 is externally the same as the 350, but has an improved cabin, reduced cabin altitude and an autothrottle system.

Challenger 600 series

The Challenger 300/350 looks most like its larger sibling, the Challenger 600 series. Look at the cockpit windows to keep them apart. They keep the same height on the 600 series, and become narrower towards the end at the 300/350. Also on all large cabin Challengers - except the 600 and 600S - the engines have a separate fan exhaust, while the Challenger 300 series have a single exhaust.

Raytheon Hawker 4000

Being similar in size you might mix-up the Challenger 300 series and the Raytheon Hawker 4000. The latter can best be distinguished by the fairings for the thrust reversers: external on the Hawker 4000, not visible on the Challenger.

Cessna 700 Citation Longitude

The Citation Longitude is also similar in size as the Challenger 300 series. To keep them apart look at the differences in cockpit windows, curved winglets and nose gear doors, apart from more subtle differences.

Embraer 550 Legacy 500

The final aircraft of similar in size is the Embraer Legacy 500/Praetor 600. The main difference is in the top of the vertical stabiliser. At the Embraer it appears to be tilted forward.