I am learning Czech, and in some courses which are very popular I have noticed that there are many wrong, or ‘weird’ or archaic words. Fortunately, I have used some translators and asked some Czech friends about those words and I ignore them. Is there any way how to notice these things more easily? Maybe I am still learning some words with the wrong definition!
Thanks!
Ah" Also, I would like to start learning Latvian, but I have no way how to |detect| possible errors on Latvian words…
I’m afraid I don’t have any ways to easily notice these types of things, but if you find errors in a course, it’s good report them on the course forum so that they can fix the issues 
One way to see if words are actually in use at all is to go to the local version of google and just type in the word. If it is a verb, you can type it in with, say, “I” or “he/she/it” in whatever tense seems appropriate, and see what you get in the search results.
I do this a lot with Swedish using google.se and I usually learn a lot just by looking at the answers that come up. I think I did discover that a few words lacked an important translation, for example, by doing this.
The other tool I use is a collocations website (sometimes known as concordance). I don’t know if such things exist for Czech and Latvian, but it is worth checking out. Sometimes it can help to find out what the relevant translations are for things like “frequency database”, or “frequency list” or “concordance” and “collocation”. There may be websites that have been created for academic purposes by linguists or dictionary publishers that might not have an internet name that is easily found.
But, basically, just google 