How can I know if I am advancing on the spiritual path? How can I know if I'm not fooling myself but really developing?

There is one clear indication for being considerably advanced on the path: it is when you don't care anymore if you have advanced or not.

It does not mean you have abandoned the spiritual path. You still have that underground pull towards the spiritual domain that keeps you on the path beyond your volition, you may even still enjoy the spiritual practices or may not, but you have become completely indifferent to whether you are progressing or not, you find yourself accepting whatever spiritual progress, non-progress or anti-progress concerned with your being. You have surrendered all objectives and targets. This is the clear indication that you are progressing, if you wish to have such an indication.

But do not turn this indication now into a new objective. It will happen when its time comes.

There are spiritual teachers who say that being calm, peaceful, relaxed is an indication to spiritual progress.

You cannot take this as an indication to progress. Sometimes on the contrary, it may indicate that you suppress hard emotions and past karma conditionings deep down in your subconscious and on the surface feels peaceful and relaxed.

Take for example the Vipassana meditation technique, the deeper you go, the stronger are the mental reaction patterns, the sanskaras, that pop up and make you more agitated. In Vipassana as well as in other certain meditations, actually it can be said that in great parts of the practice, feeling agitated is a good sign of progress.

Is there anything I can do to make sure I will progress

Yes, always be sincere with yourself.

You are always in progress. You may make the progress more speedy and efficient by always being sure that you are sincere with yourself. When your mind deceives you, it, of course, can make the path longer, longer even in extra lives, this is an objective of the mind: to delay your progress forever because spiritual progress means for the mind one thing: its extinction, or more accurately, the extinction of its control over you.

This is the case, by the way, where a guru may be most beneficial by spotting in real time when you are deceiving yourself and showing it to you. This is the main role of a guru.

I may wander on the path for eons and not progress

Who is this "I" that worries about wandering for eons on the path? Search for it.

You look at the spiritual path from your personality's skeptical point of view as if the trail of the path is chosen by you and is limited only to the official satsangs, meditations, reading spiritual books etc. Your spiritual path is always there, it is there also when you go to work or walk with your dog or crawl with a neighbor. Just embark on the path in the same way you step on an escalator, leave aside worries about progress and instead always be alert to make sure you are sincere with yourself, always check on yourself like a suspicious petty employer.

(From verbatim of Talks with Riktam Kantu, Vol. A)