If you have replaced the front struts in most modern cars, you will have disconnected one or more components that control the alignment, such as tie rods or control arms (depending on the vehicle). Reattaching these in precisely the same position is almost impossible (although you can get pretty close), and that's assuming your alignment was 100% correct before replacing the struts.
is there anything that I can diagnose ... as to whether or not an alignment is needed and what kind of alignment is needed?
If you replaced the front struts, an alignment is needed.
You may not suffer any significant short term symptoms from not getting one, but you will have additional wear and tear on the car (particularly suspension and tires) and handling may suffer.
Significant alignment problems will show up as things such as the steering wheel being misaligned, or noticeable pull one direction or another. Other strange handling characteristics (taken with a general knowledge or assessment of the condition of the car to rule out other causes) can also point to an alignment issue, but these vary and it would be hard to pinpoint a specific thing to look for. (More of a question of "does it handle differently after the repair?")
If you don't have noticeable handling issues, you may be fine driving your car for a while until it's convenient to get an alignment (as opposed to taking it directly to a shop after the repair), but be aware if things are misaligned you could be causing damage. (I personally would probably feel comfortable doing this for a few weeks if everything felt fine, but your risk is your own.)
As to what kind of alignment is needed, these days a shop with modern equipment will put devices on all four wheels that reflect at various angles into sensors and they will tell you whether you need an alignment on all four wheels. However, if you know that your alignment is already 100% correct before the repair, you don't need to worry about the back wheels when replacing the front struts only.
