Comprobamos la prueba fotográfica por si dudamos.

Comprobamos is the 1st person plural of comprobar in the present indicative.

It usually means:

• we check
• we verify
• we confirm

In this sentence, we check or we verify is the most natural translation.

The present tense in Spanish can describe:

• something happening now
• a habitual action
• a general procedure

So Comprobamos la prueba fotográfica... can mean something like:

• We check the photographic evidence
• We usually check the photographic evidence
• We check the photographic evidence as a precaution

It does not have to mean only right now at this exact moment.

Prueba can mean several things in Spanish, including:

• test
• proof
• evidence
• trial

Here, because of fotográfica, it most likely means photographic evidence or photographic proof, not a photographic test.

This is common in Spanish: one word can cover meanings that English separates more clearly.

In Spanish, adjectives often come after the noun, especially when they classify the noun rather than describe it emotionally or subjectively.

So:

• prueba fotográfica = photographic evidence
• literally, evidence photographic

Here fotográfica tells you what kind of evidence it is.

Por si means in case or just in case.

It introduces a precaution: you do one thing because another thing might happen.

So the structure is:

• Hacemos X por si pasa Y
• We do X in case Y happens

In this sentence:

• Comprobamos la prueba fotográfica por si dudamos
• We check the photographic evidence in case we have doubts

This is a very common question.

• si = if
• por si = in case

Compare:

• Si dudamos, comprobamos la prueba fotográfica.
  = If we have doubts, we check the photographic evidence.

• Comprobamos la prueba fotográfica por si dudamos.
  = We check the photographic evidence in case we have doubts.

With si, the doubt is the condition for checking.

With por si, the checking is done as a precaution before or in preparation for possible doubt.

Because por si normally takes the indicative, not the subjunctive.

That surprises many English speakers, because the idea is uncertain. But after por si, Spanish usually uses the indicative to refer to a possible situation you are preparing for.

Examples:

• Lleva un paraguas por si llueve.
  = Take an umbrella in case it rains.

• Guardé dinero por si lo necesitaba.
  = I saved money in case I needed it.

So:

• por si dudamos is the expected pattern
• por si dudemos would not be standard here

More or less, yes, but not always in exactly the same way.

Dudar can mean:

• to doubt
• to be unsure
• to hesitate

In this sentence, dudamos is understood as we have doubts or we are unsure.

English often prefers a phrase like have doubts, and Spanish often does too. That is why many native speakers might naturally say:

• por si tenemos dudas

instead of:

• por si dudamos

Both are understandable, but tenemos dudas can sound a bit more natural in many contexts.

Because dudar can be used without an expressed object.

You can say:

• Dudamos. = We have doubts / We’re unsure.

Or you can specify what the doubt is about:

• Dudamos de su versión. = We doubt his/her version.
• Dudamos de los datos. = We doubt the data.

In your sentence, what is being doubted is left implicit, because the context already suggests it.

La suggests that the photographic evidence is specific or already known in the context.

So:

• la prueba fotográfica = the photographic evidence
• una prueba fotográfica = some photographic evidence / a piece of photographic evidence

Spanish often uses the definite article when the speaker assumes the listener knows what is being referred to.

It is understandable and grammatically workable, but on its own it can sound a little unusual.

A native speaker might more naturally say:

• Comprobamos la prueba fotográfica por si tenemos dudas.
• Comprobamos la prueba fotográfica por si acaso.
• Comprobamos la prueba fotográfica en caso de duda.

Why? Because dudamos feels slightly bare here. Spanish often prefers tener dudas in this kind of situation.

So the original sentence is useful for learning por si + indicative, but in everyday Spanish, a small rewording might sound more idiomatic.