Is there a way to tell who will be next in combat?

Units with higher initiative move move first.

Try holding CTRL (or ALT, I don't remember which), it should display hp bars and numbers that signify initiative order.

Like in this screen: http://www.gamer.ru/system/attached_images/images/000/371/354/original/screen_110522_002.jpg

Thanks - the blue numbers are the initiative order?

Nope, blue number is an amount of units in a group.
If you haven't figured it out, it can be enabled and disabled by pressing ALT, not holding.

It's the number to the right that shows who plays when. The lower and greener the number, the sooner that stack will play. When there is "-" instead of number it means the stack has already acted in this turn.

On picture attached by Hrymr, there is no no. 1 as that unit is just acting when creen was captured. So next to act would be that single unit next to enemy elven archers, then rune mages. Beholders and red dragons already acted and last to act are the ancient ents on the left with the orange number 7 (it goes from green to red shades).

Every unit counterattacks once per turn (only in meele, no ranged counterattacks).
Expections:
- Unlimited counterattack (griffons) - always counterattacks, unless attacked by unit with "No retalation" abilty.
- No retalation (snakes, vampires) - attacks without causing retaliation.

Thanks again