What are some signs that my horse is going to foal soon?

This is our first foal, (She's an old pro at it however). This will be her 4th foal in as many years (I just bought her last fall, with last springs filly). Her last foal was May 29th of last year. So should be expecting anything to happen this month, or could It go into May-June? She was exposed to the stud from June 1st - August1st last year. I'm looking at it being May but she's just so freaking huge!!! Her shape has changed recently & her udder is Huge, with thick veins running the inside of her leg/groin area. I had vetted the end of Nov. & he said she was most likely 5 mo.... I just want to know some signs to look out for. Any tips from old pros would be appreciated. Our vet is only minutes away if complications arise. Thanks : )