When Julia finds Tom unconscious in the desert, she gets out of the car and shouts Tom's name several times as she runs towards him. Listen carefully, and you'll hear Tilda Swinton calling out 'Tom' with her real life English accent instead of her character's American accent.
The motel scenes in which Julia holds Tom captive that are supposed to be set in California have non-American electrical outlets, revealing the scenes' Mexican location.
After Julia goes to the bus station to retrieve the money, she steals a car after noticing policemen keeping guard next to hers. When she arrives in the desert and finds Tom unconscious, she wakes him up by splashing him with water from the bottle that was in her original car.
Toward the end of the film, Julia tries to coax Tom to cross from the highway median to her side but traffic doesn't slow to allow this to happen. She could have used her gun to convince traffic to slow. Also, a passing car knocks the gun from her hand, which seems incredibly unlikely.