At the moment you simply have suspicions. If your h runs true to form, if/when you confront him, he may well try to convince you that you've fallen victim to pg hormones and imagined the whole thing - and you'll start doubting yourself.
How's your acting skills? Go to h's place of work c11am one morning claiming that you came out carrying your purse (i.e you had the money to get to his office) but left your keys (hide them in a pocket) in your handbag together with your mobile (you couldn't call to let him know you were on your way) and need his keys to get back in the house.
With luck you may bump into the OW who may, or may not, be aware of the impending birth of his dc2. Don't worry if she isn't present; word will get back to her. Go on a charm offensive with as many of h's colleagues as possible- 'we' can't wait to welcome our new addition, 'darling h was so wonderful last time round', 'darling h' is a brilliant df to dd and our new addition will have a doting df' gush, gush, blah de sickening blah.
Smile sweetly throughout, linger as long as you can - maybe a slight dizziness necessitating a cup of tea etc? Dress smartly casual for the occasion - think yummy mummy with bump. Light daytime make up but lippy is a must and your cheeks should have a blooming glow even you have to paint it on with rosy pink blusher.
In any event, I strongly advise you not to show your hand or voice your suspicions and continue to watch and wait. If he should announce that he's enamoured with another woman you can completely take the wind out of his sails by coolly and calmly telling him 'I know you've been playing me for an idiot for months but, as the saying goes, the yolk's on you'.
'I instructed a solicitor some weeks ago and I'll be filing for divorce citing your adultery - btw, tell your OW that she'll have her 5 minutes of fame - and unreasonable behaviour. Of course the dc and myself will continue to live here and my solicitor will be in touch with you regarding maintenance and other matters'.
'There's nothing further to discuss and I suggest you now go and pack what you need and leave'.
Repeat the last sentence as often as necessary and keep up the enigmatic front until he's out of the door at which time you can vent.
Keep your self-respect and dignity at all times and don't let the bastard grind you down because, if he's done what you suspect him of doing, he's nothing but a lowlife piece of shit.