I had my suspicions, ds had been distant, never put his phone down.
There was one person I had particular suspicions about, when I realised her fb was set to public I thought it was Christmas. I was trawling through her posts when one stood out, she'd tagged herself in at an arena, to see a band on the EXACT same night as dp saw them, dp told me he was going with someone at work but they'd let him down (we couldn't go together due to lack of a babysitter), told me someone was just hanging about outside the arena and he managed to sell the other ticket to that person... 
Once I found this fb post, I realised that I'd emptied dp jean pockets earlier that week to wash them, with I presumed was his ticket stub which I'd thrown in our upstairs bin, I retrieved them and of course there were two ticket stubs, which put paid to the fact he'd sold the ticket to someone lurking outside the gig!
I asked him if he'd seen this woman at the venue, the bar areas are quite small so you usually see someone you know milling about before hand. Nope he hadn't seen her, I mentioned her fb post (at this point all of my worries about anyone finding out I was fb stalking had gone out of the window) and I also wondered why (as at that time he was always on fb) he hadn't commented on the fact that they'd both been to see the same band, on the same night, at the same place. He denied EVERYTHING. He said he walked into the venue with the person he sold the ticket to and kept the stub. He denied denied denied.
I thought I was going insane, I KNEW something was amiss.
It all came to ahead the following February. It all caught up with him, all the lies etc, he only then told me everything, oh and of course he had been to the gig with this woman! In fact he'd met her and two other women during the previous year (that's a whole other story), he'd been a busy boy! She was also saved under a mans name on his phone along with two other women.
I think if you're instincts are screaming at you that something isn't right, you're usually spot on.
Sorry for the ramble.