Not specifically a sign or signs but for everything you think, feel, consider, or are told to do, keep in mind that:
1. People Lie
2. People Fuck
Those two facts are as certain as death and taxes.
Signs--There are many and I don't know or can remember them all.
Perhaps a recent loss of interest in you, ESPECIALLY in matters of sex. A loss of enthusiasm for doing things 'together'.
Working later hours, working on 'projects' that take personal time and/or interaction with coworkers.
Sneaking private time, maybe even getting caught texting/sexting and trying to cover up what she is doing.
'Mysterious' phone calls or calls she walks away to take.
Recent or frequent "nights out", staying out late or overnight.
Changing hairstyle, makeup, and/or wardrobe. Especially if new lingerie is being acquired.
Being told "we're just friends".
Does any of that match what your wife is doing?