Ok, there should be a windsock somewhere. Just take a look and see which way it is pointing relative to the runway, and if it is stable or lifting up and down. If it is moving a lot and at 90 degrees to the runway, and you are having to crab such that you are looking at the runway through the window at the side, then that is enough crosswind to land on a more into wind runway, or another airport.
No maths required.
Remember, you have two things to consider, direction and strength. Light wind at 90 degrees, no problem. Strong or gusting wind, you will have windshear to consider as well, and possibly difficulty taxi-ing. So that is why you calculate crosswind component WHEN PLANNING THE FLIGHT.
Not during the approach. You should leave the ground with a forecast that shows you landing where conditions are within your capabilities. Plan B comes into play when things change, weather forecasts aren't 100% after all.
On the approach it's all about sight picture and (dare I say this) feeling. Set up your approach at a comfortable height, flaps configured for landing, trimmed for the airspeed you have previously calculated. Use flight manual figures please. Make due allowance for gusts.
Then look at what is happening outside, as well as inside.
If the crab angle (or wing down, if you do it that way) is reasonable, and you are staying lined up with the runway, then carry on with an approach, with the mindset that you will be going around, and might convert to a landing if all goes perfectly. This way you get to see exactly what is happening before committing to a landing.
If you do this and are unhappy with the approach, you can always have another go. If it still isn't looking good, a different runway or diversion may be in order. You can always ask for a more suitable runway if you, the Captain, consider that is what you need.
As you gain experience you will be able to cope with more demanding conditions.
Practice with your instructor if you feel you are outside your comfort zone when flying solo.
If you do decide to land, fly the aircraft until it stops.
If you are flying a tailwheel type, fly the aircraft until it is tied down.