Reconnaissance and tactical intelligence is one of the most vital elements of strategy in Sudden Strike 4. Without a good overlook on the battlefield, your tanks may quickly yield even to a small unit of infantry/artillery/cannons. The infantry provides the best vision, so in every attack it should either advance before or together with tanks. However, you should still learn what you're up against beforehand. There are a couple of ways to do that.
The simplest and most effective way is to designate one soldier as scout - ideally, this would be a sniper or an officer. Engage cease-fire, order prone stance and advance towards the enemy. If you're getting close to a tree line or buildings, order him to move in small leaps. This will allow him to survive should he encounter a building occupied by enemies and call an artillery strike. The officer is at an advantage, thanks to the Binoculars. They allow him to spot enemies hidden in vegetation or buildings from afar.
If you have few units of infantry left, you can use a light vehicle such as. BA-10 (Russia), M8 Greyhound (Allies), Sdkfz 222 or Puma (Germans). Ideally, you want to have the Camouflage skill available. It will allow you to stay unnoticed using vegetation and approach the enemy. However, these vehicles are available only in a couple of missions. Same goes for reconnaissance aircraft - it's only available in a handful of missions. They will reaveal the enemy forces in a part of the map, allowing you to fire at them. It may seem counter-intuitive, but the aircraft should be used directly before the battle. This will allow you to destroy enemy tanks and artillery from a safe distance without putting your units at risk.
You can also try a tactic called Reconnaissance by fire. The optimal agent here is a medium tank, which guarantees reliable firepower and at the same time good speed. It won't give you a full recon, but it should produce a response of the enemy, which should allow you to estimate their numbers and firepower (projectiles with a straight trajectory indicate infantry and armor; those with a parable indicate artillery). Hatch lookout or Tank Periscopes of the armor doctrine can significantly improve your recon performance. Try not to stop in one spot; reverse or drive to the sides; use buildings and vegetation for cover.