How can you tell a V10 from a V8 - by looking at the car?

 

if u mean the R8, the V10 upgrades are:

dual oval exhaust outlets
bigger side blades
new mirror housings
blackened front and rear air vents with fewer strakes
revised rocker panels for increased downforce
larger brakes
exclusive wheels
LED headlights
rear diffuser
V10 gauges and gear stick
V10 badges on fenders
chrome grill stripes
V10 engine with carbon fiber surround
rear window exhaust grilles in aluminium look
chrome plated left foot rest